diff --git a/clang/include/clang/Basic/Features.def b/clang/include/clang/Basic/Features.def
--- a/clang/include/clang/Basic/Features.def
+++ b/clang/include/clang/Basic/Features.def
@@ -278,6 +278,7 @@
 EXTENSION(matrix_types_scalar_division, true)
 EXTENSION(cxx_attributes_on_using_declarations, LangOpts.CPlusPlus11)
 
+FEATURE(builtin_headers_in_system_modules, LangOpts.BuiltinHeadersInSystemModules)
 FEATURE(cxx_abi_relative_vtable, LangOpts.CPlusPlus && LangOpts.RelativeCXXABIVTables)
 
 // CUDA/HIP Features
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -173,6 +173,7 @@
 BENIGN_LANGOPT(HeinousExtensions , 1, 0, "extensions that we really don't like and may be ripped out at any time")
 LANGOPT(Modules           , 1, 0, "modules semantics")
 COMPATIBLE_LANGOPT(CPlusPlusModules, 1, 0, "C++ modules syntax")
+LANGOPT(BuiltinHeadersInSystemModules, 1, 0, "builtin headers belong to system modules, and _Builtin_ modules are ignored for cstdlib headers")
 BENIGN_ENUM_LANGOPT(CompilingModule, CompilingModuleKind, 3, CMK_None,
                     "compiling a module interface")
 BENIGN_LANGOPT(CompilingPCH, 1, 0, "building a pch")
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2946,6 +2946,13 @@
           "Enable the 'modules' language feature">,
   NegFlag<SetFalse>, BothFlags<
           [NoXarchOption], [ClangOption, CLOption]>>;
+def fbuiltin_headers_in_system_modules : Flag <["-"], "fbuiltin-headers-in-system-modules">,
+  Group<f_Group>,
+  Flags<[NoXarchOption]>,
+  Visibility<[CC1Option]>,
+  ShouldParseIf<fmodules.KeyPath>,
+  HelpText<"builtin headers belong to system modules, and _Builtin_ modules are ignored for cstdlib headers">,
+  MarshallingInfoFlag<LangOpts<"BuiltinHeadersInSystemModules">>;
 def fmodule_maps : Flag <["-"], "fmodule-maps">,
   Visibility<[ClangOption, CLOption]>, Alias<fimplicit_module_maps>;
 def fmodule_name_EQ : Joined<["-"], "fmodule-name=">, Group<f_Group>,
diff --git a/clang/include/clang/Lex/ModuleMap.h b/clang/include/clang/Lex/ModuleMap.h
--- a/clang/include/clang/Lex/ModuleMap.h
+++ b/clang/include/clang/Lex/ModuleMap.h
@@ -415,7 +415,6 @@
   }
 
   /// Is this a compiler builtin header?
-  static bool isBuiltinHeader(StringRef FileName);
   bool isBuiltinHeader(const FileEntry *File);
 
   /// Add a module map callback.
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -2846,6 +2846,25 @@
   return TargetVersion < alignedAllocMinVersion(OS);
 }
 
+static bool sdkSupportsBuiltinModules(const Darwin::DarwinPlatformKind &TargetPlatform, const std::optional<DarwinSDKInfo> &SDKInfo) {
+  if (!SDKInfo)
+    return false;
+
+  VersionTuple SDKVersion = SDKInfo->getVersion();
+  switch (TargetPlatform) {
+  case Darwin::MacOS:
+    return SDKVersion >= VersionTuple(99U);
+  case Darwin::IPhoneOS:
+    return SDKVersion >= VersionTuple(99U);
+  case Darwin::TvOS:
+    return SDKVersion >= VersionTuple(99U);
+  case Darwin::WatchOS:
+    return SDKVersion >= VersionTuple(99U);
+  default:
+    return true;
+  }
+}
+
 void Darwin::addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
                                    llvm::opt::ArgStringList &CC1Args,
                                    Action::OffloadKind DeviceOffloadKind) const {
@@ -2868,6 +2887,20 @@
           options::OPT_fvisibility_inlines_hidden_static_local_var,
           options::OPT_fno_visibility_inlines_hidden_static_local_var))
     CC1Args.push_back("-fvisibility-inlines-hidden-static-local-var");
+
+  // Earlier versions of the darwin SDK have the C standard library headers
+  // all together in the Darwin module. That leads to module cycles with
+  // the _Builtin_ modules. e.g. <inttypes.h> on darwin includes <stdint.h>.
+  // The builtin <stdint.h> include-nexts <stdint.h>. When both of those
+  // darwin headers are in the Darwin module, there's a module cycle Darwin ->
+  // _Builtin_stdint -> Darwin (i.e. inttypes.h (darwin) -> stdint.h (builtin) ->
+  // stdint.h (darwin)). This is fixed in later versions of the darwin SDK,
+  // but until then, the builtin headers need to join the system modules.
+  // i.e. when the builtin stdint.h is in the Darwin module too, the cycle
+  // goes away. Note that -fbuiltin-headers-in-system-modules does nothing
+  // to fix the same problem with C++ headers, and is generally fragile.
+  if (!sdkSupportsBuiltinModules(TargetPlatform, SDKInfo))
+    CC1Args.push_back("-fbuiltin-headers-in-system-modules");
 }
 
 void Darwin::addClangCC1ASTargetOptions(
diff --git a/clang/lib/Headers/__stdarg___gnuc_va_list.h b/clang/lib/Headers/__stdarg___gnuc_va_list.h
--- a/clang/lib/Headers/__stdarg___gnuc_va_list.h
+++ b/clang/lib/Headers/__stdarg___gnuc_va_list.h
@@ -7,7 +7,8 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef __GNUC_VA_LIST
+/* See stdarg.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(__GNUC_VA_LIST) || __has_feature(builtin_headers_in_system_modules)
 #define __GNUC_VA_LIST
 typedef __builtin_va_list __gnuc_va_list;
 #endif
diff --git a/clang/lib/Headers/__stdarg___va_copy.h b/clang/lib/Headers/__stdarg___va_copy.h
--- a/clang/lib/Headers/__stdarg___va_copy.h
+++ b/clang/lib/Headers/__stdarg___va_copy.h
@@ -7,6 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef __va_copy
+/* See stdarg.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(__va_copy) || __has_feature(builtin_headers_in_system_modules)
 #define __va_copy(d, s) __builtin_va_copy(d, s)
 #endif
diff --git a/clang/lib/Headers/__stdarg_va_arg.h b/clang/lib/Headers/__stdarg_va_arg.h
--- a/clang/lib/Headers/__stdarg_va_arg.h
+++ b/clang/lib/Headers/__stdarg_va_arg.h
@@ -7,7 +7,8 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef va_arg
+/* See stdarg.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(va_arg) || __has_feature(builtin_headers_in_system_modules)
 
 #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 /* C23 does not require the second parameter for va_start. */
diff --git a/clang/lib/Headers/__stdarg_va_copy.h b/clang/lib/Headers/__stdarg_va_copy.h
--- a/clang/lib/Headers/__stdarg_va_copy.h
+++ b/clang/lib/Headers/__stdarg_va_copy.h
@@ -7,6 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef va_copy
+/* See stdarg.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(va_copy) || __has_feature(builtin_headers_in_system_modules)
 #define va_copy(dest, src) __builtin_va_copy(dest, src)
 #endif
diff --git a/clang/lib/Headers/__stdarg_va_list.h b/clang/lib/Headers/__stdarg_va_list.h
--- a/clang/lib/Headers/__stdarg_va_list.h
+++ b/clang/lib/Headers/__stdarg_va_list.h
@@ -7,7 +7,8 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef _VA_LIST
+/* See stdarg.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(_VA_LIST) || __has_feature(builtin_headers_in_system_modules)
 #define _VA_LIST
 typedef __builtin_va_list va_list;
 #endif
diff --git a/clang/lib/Headers/__stddef_max_align_t.h b/clang/lib/Headers/__stddef_max_align_t.h
--- a/clang/lib/Headers/__stddef_max_align_t.h
+++ b/clang/lib/Headers/__stddef_max_align_t.h
@@ -1,4 +1,4 @@
-/*===---- __stddef_max_align_t.h - Definition of max_align_t for modules ---===
+/*===---- __stddef_max_align_t.h - Definition of max_align_t ---------------===
  *
  * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  * See https://llvm.org/LICENSE.txt for license information.
@@ -7,7 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef __CLANG_MAX_ALIGN_T_DEFINED
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(__CLANG_MAX_ALIGN_T_DEFINED) ||                                   \
+    __has_feature(builtin_headers_in_system_modules)
 #define __CLANG_MAX_ALIGN_T_DEFINED
 
 #if defined(_MSC_VER)
diff --git a/clang/lib/Headers/__stddef_nullptr_t.h b/clang/lib/Headers/__stddef_nullptr_t.h
--- a/clang/lib/Headers/__stddef_nullptr_t.h
+++ b/clang/lib/Headers/__stddef_nullptr_t.h
@@ -7,11 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_NULLPTR_T) || __has_feature(modules)
-/* Always define nullptr_t when modules are available. */
-#if !__has_feature(modules)
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(_NULLPTR_T) || __has_feature(builtin_headers_in_system_modules)
 #define _NULLPTR_T
-#endif
+
 #ifdef __cplusplus
 #if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
 namespace std {
@@ -22,4 +21,5 @@
 #else
 typedef typeof(nullptr) nullptr_t;
 #endif
+
 #endif
diff --git a/clang/lib/Headers/__stddef_offsetof.h b/clang/lib/Headers/__stddef_offsetof.h
--- a/clang/lib/Headers/__stddef_offsetof.h
+++ b/clang/lib/Headers/__stddef_offsetof.h
@@ -7,7 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(offsetof) || __has_feature(modules)
-/* Always define offsetof when modules are available. */
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(offsetof) || __has_feature(builtin_headers_in_system_modules)
 #define offsetof(t, d) __builtin_offsetof(t, d)
 #endif
diff --git a/clang/lib/Headers/__stddef_ptrdiff_t.h b/clang/lib/Headers/__stddef_ptrdiff_t.h
--- a/clang/lib/Headers/__stddef_ptrdiff_t.h
+++ b/clang/lib/Headers/__stddef_ptrdiff_t.h
@@ -7,10 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_PTRDIFF_T) || __has_feature(modules)
-/* Always define ptrdiff_t when modules are available. */
-#if !__has_feature(modules)
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(_PTRDIFF_T) || __has_feature(builtin_headers_in_system_modules)
 #define _PTRDIFF_T
-#endif
+
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
 #endif
diff --git a/clang/lib/Headers/__stddef_rsize_t.h b/clang/lib/Headers/__stddef_rsize_t.h
--- a/clang/lib/Headers/__stddef_rsize_t.h
+++ b/clang/lib/Headers/__stddef_rsize_t.h
@@ -7,10 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_RSIZE_T) || __has_feature(modules)
-/* Always define rsize_t when modules are available. */
-#if !__has_feature(modules)
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(_RSIZE_T) || __has_feature(builtin_headers_in_system_modules)
 #define _RSIZE_T
-#endif
+
 typedef __SIZE_TYPE__ rsize_t;
+
 #endif
diff --git a/clang/lib/Headers/__stddef_size_t.h b/clang/lib/Headers/__stddef_size_t.h
--- a/clang/lib/Headers/__stddef_size_t.h
+++ b/clang/lib/Headers/__stddef_size_t.h
@@ -7,10 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_SIZE_T) || __has_feature(modules)
-/* Always define size_t when modules are available. */
-#if !__has_feature(modules)
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(_SIZE_T) || __has_feature(builtin_headers_in_system_modules)
 #define _SIZE_T
-#endif
+
 typedef __SIZE_TYPE__ size_t;
+
 #endif
diff --git a/clang/lib/Headers/__stddef_unreachable.h b/clang/lib/Headers/__stddef_unreachable.h
--- a/clang/lib/Headers/__stddef_unreachable.h
+++ b/clang/lib/Headers/__stddef_unreachable.h
@@ -7,7 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(unreachable) || __has_feature(modules)
-/* Always define unreachable when modules are available. */
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(unreachable) || __has_feature(builtin_headers_in_system_modules)
 #define unreachable() __builtin_unreachable()
 #endif
diff --git a/clang/lib/Headers/__stddef_wchar_t.h b/clang/lib/Headers/__stddef_wchar_t.h
--- a/clang/lib/Headers/__stddef_wchar_t.h
+++ b/clang/lib/Headers/__stddef_wchar_t.h
@@ -8,14 +8,17 @@
  */
 
 #if !defined(__cplusplus) || (defined(_MSC_VER) && !_NATIVE_WCHAR_T_DEFINED)
-/* Always define wchar_t when modules are available. */
-#if !defined(_WCHAR_T) || __has_feature(modules)
-#if !__has_feature(modules)
+
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(_WCHAR_T) || __has_feature(builtin_headers_in_system_modules)
 #define _WCHAR_T
-#if defined(_MSC_EXTENSIONS)
+
+#ifdef _MSC_EXTENSIONS
 #define _WCHAR_T_DEFINED
 #endif
-#endif
+
 typedef __WCHAR_TYPE__ wchar_t;
+
 #endif
+
 #endif
diff --git a/clang/lib/Headers/__stddef_wint_t.h b/clang/lib/Headers/__stddef_wint_t.h
--- a/clang/lib/Headers/__stddef_wint_t.h
+++ b/clang/lib/Headers/__stddef_wint_t.h
@@ -7,10 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-/* Always define wint_t when modules are available. */
-#if !defined(_WINT_T) || __has_feature(modules)
-#if !__has_feature(modules)
+/* See stddef.h for an explanation of builtin_headers_in_system_modules. */
+#if !defined(_WINT_T) || __has_feature(builtin_headers_in_system_modules)
 #define _WINT_T
-#endif
+
 typedef __WINT_TYPE__ wint_t;
+
 #endif
diff --git a/clang/lib/Headers/stdarg.h b/clang/lib/Headers/stdarg.h
--- a/clang/lib/Headers/stdarg.h
+++ b/clang/lib/Headers/stdarg.h
@@ -7,9 +7,37 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(__STDARG_H) || defined(__need___va_list) ||                       \
-    defined(__need_va_list) || defined(__need_va_arg) ||                       \
-    defined(__need___va_copy) || defined(__need_va_copy)
+/*
+ * This header is designed to be included multiple times. If one of the __need_
+ * macros are defined, then only that subset of interfaces are provided. This
+ * can be useful for POSIX headers that need to not expose all of stdarg.h, but
+ * need to use some of its interfaces. Otherwise this header provides all of
+ * the expected interfaces.
+ *
+ * When clang modules are enabled, this header is a textual header. It ignores
+ * its header guard so that multiple submodules can export its interfaces.
+ * Take module SM with submodules A and B, whose headers both include stdarg.h
+ * When SM.A builds, __STDARG_H will be defined. When SM.B builds, the
+ * definition from SM.A will leak when building without local submodule
+ * visibility. stdarg.h wouldn't include any of its implementation headers, and
+ * SM.B wouldn't import any of the stdarg modules, and SM.B's `export *`
+ * wouldn't export any stdarg interfaces as expected. However, since stdarg.h
+ * ignores its header guard when building with modules, it all works as
+ * expected.
+ *
+ * When clang modules are enabled, the implementation headers are not textual
+ * headers, so they need header guards. The exception is when the
+ * builtin_headers_in_system_modules feature is active. In that situation, the
+ * implementation headers are non-modular, and so need to behave like textual
+ * headers without header guards.
+ *
+ * When clang modules are not enabled, none of this special behavior is
+ * required, and the header guards can function in the normal simple fashion.
+ */
+#if !defined(__STDARG_H) || __has_feature(modules) ||                          \
+    defined(__need___va_list) || defined(__need_va_list) ||                    \
+    defined(__need_va_arg) || defined(__need___va_copy) ||                     \
+    defined(__need_va_copy)
 
 #if !defined(__need___va_list) && !defined(__need_va_list) &&                  \
     !defined(__need_va_arg) && !defined(__need___va_copy) &&                   \
diff --git a/clang/lib/Headers/stddef.h b/clang/lib/Headers/stddef.h
--- a/clang/lib/Headers/stddef.h
+++ b/clang/lib/Headers/stddef.h
@@ -7,23 +7,47 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(__STDDEF_H) || defined(__need_ptrdiff_t) ||                       \
-    defined(__need_size_t) || defined(__need_rsize_t) ||                       \
-    defined(__need_wchar_t) || defined(__need_NULL) ||                         \
-    defined(__need_nullptr_t) || defined(__need_unreachable) ||                \
-    defined(__need_max_align_t) || defined(__need_offsetof) ||                 \
-    defined(__need_wint_t) ||                                                  \
-    (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1)
+/*
+ * This header is designed to be included multiple times. If one of the __need_
+ * macros are defined, then only that subset of interfaces are provided. This
+ * can be useful for POSIX headers that need to not expose all of stddef.h, but
+ * need to use some of its interfaces. Otherwise this header provides all of
+ * the expected interfaces.
+ *
+ * When clang modules are enabled, this header is a textual header. It ignores
+ * its header guard so that multiple submodules can export its interfaces.
+ * Take module SM with submodules A and B, whose headers both include stddef.h
+ * When SM.A builds, __STDDEF_H will be defined. When SM.B builds, the
+ * definition from SM.A will leak when building without local submodule
+ * visibility. stddef.h wouldn't include any of its implementation headers, and
+ * SM.B wouldn't import any of the stddef modules, and SM.B's `export *`
+ * wouldn't export any stddef interfaces as expected. However, since stddef.h
+ * ignores its header guard when building with modules, it all works as
+ * expected.
+ *
+ * When clang modules are enabled, the implementation headers are not textual
+ * headers, so they need header guards. The exception is when the
+ * builtin_headers_in_system_modules feature is active. In that situation, the
+ * implementation headers are non-modular, and so need to behave like textual
+ * headers without header guards.
+ *
+ * When clang modules are not enabled, none of this special behavior is
+ * required, and the header guards can function in the normal simple fashion.
+ */
+#if !defined(__STDDEF_H) || __has_feature(modules) ||                          \
+    (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1) ||        \
+    defined(__need_ptrdiff_t) || defined(__need_size_t) ||                     \
+    defined(__need_rsize_t) || defined(__need_wchar_t) ||                      \
+    defined(__need_NULL) || defined(__need_nullptr_t) ||                       \
+    defined(__need_unreachable) || defined(__need_max_align_t) ||              \
+    defined(__need_offsetof) || defined(__need_wint_t)
 
 #if !defined(__need_ptrdiff_t) && !defined(__need_size_t) &&                   \
     !defined(__need_rsize_t) && !defined(__need_wchar_t) &&                    \
     !defined(__need_NULL) && !defined(__need_nullptr_t) &&                     \
     !defined(__need_unreachable) && !defined(__need_max_align_t) &&            \
     !defined(__need_offsetof) && !defined(__need_wint_t)
-/* Always define miscellaneous pieces when modules are available. */
-#if !__has_feature(modules)
 #define __STDDEF_H
-#endif
 #define __need_ptrdiff_t
 #define __need_size_t
 /* ISO9899:2011 7.20 (C11 Annex K): Define rsize_t if __STDC_WANT_LIB_EXT1__ is
diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -253,6 +253,45 @@
   return NormalHdrFile;
 }
 
+/// Determine whether the given file name is the name of a builtin
+/// header, supplied by Clang to replace, override, or augment existing system
+/// headers.
+static bool isBuiltinHeaderName(StringRef FileName) {
+  return llvm::StringSwitch<bool>(FileName)
+           .Case("float.h", true)
+           .Case("iso646.h", true)
+           .Case("limits.h", true)
+           .Case("stdalign.h", true)
+           .Case("stdarg.h", true)
+           .Case("stdatomic.h", true)
+           .Case("stdbool.h", true)
+           .Case("stddef.h", true)
+           .Case("stdint.h", true)
+           .Case("tgmath.h", true)
+           .Case("unwind.h", true)
+           .Default(false);
+}
+
+/// Determine whether the given module name is the name of a builtin
+/// module that is cyclic with a system module.
+static bool isBuiltInModuleName(StringRef ModuleName) {
+  return llvm::StringSwitch<bool>(ModuleName)
+           .Case("_Builtin_float", true)
+           .Case("_Builtin_inttypes", true)
+           .Case("_Builtin_iso646", true)
+           .Case("_Builtin_limits", true)
+           .Case("_Builtin_stdalign", true)
+           .Case("_Builtin_stdarg", true)
+           .Case("_Builtin_stdatomic", true)
+           .Case("_Builtin_stdbool", true)
+           .Case("_Builtin_stddef", true)
+           .Case("_Builtin_stdint", true)
+           .Case("_Builtin_stdnoreturn", true)
+           .Case("_Builtin_tgmath", true)
+           .Case("_Builtin_unwind", true)
+           .Default(false);
+}
+
 void ModuleMap::resolveHeader(Module *Mod,
                               const Module::UnresolvedHeaderDirective &Header,
                               bool &NeedsFramework) {
@@ -297,7 +336,7 @@
       llvm::sys::path::is_absolute(Header.FileName) ||
       Mod->isPartOfFramework() || !Mod->IsSystem || Header.IsUmbrella ||
       !BuiltinIncludeDir || BuiltinIncludeDir == Mod->Directory ||
-      !isBuiltinHeader(Header.FileName))
+      !LangOpts.BuiltinHeadersInSystemModules || !isBuiltinHeaderName(Header.FileName))
     return false;
 
   // This is a system module with a top-level header. This header
@@ -373,28 +412,9 @@
   return Name;
 }
 
-/// Determine whether the given file name is the name of a builtin
-/// header, supplied by Clang to replace, override, or augment existing system
-/// headers.
-bool ModuleMap::isBuiltinHeader(StringRef FileName) {
-  return llvm::StringSwitch<bool>(FileName)
-           .Case("float.h", true)
-           .Case("iso646.h", true)
-           .Case("limits.h", true)
-           .Case("stdalign.h", true)
-           .Case("stdarg.h", true)
-           .Case("stdatomic.h", true)
-           .Case("stdbool.h", true)
-           .Case("stddef.h", true)
-           .Case("stdint.h", true)
-           .Case("tgmath.h", true)
-           .Case("unwind.h", true)
-           .Default(false);
-}
-
 bool ModuleMap::isBuiltinHeader(const FileEntry *File) {
-  return File->getDir() == BuiltinIncludeDir &&
-         ModuleMap::isBuiltinHeader(llvm::sys::path::filename(File->getName()));
+  return File->getDir() == BuiltinIncludeDir && LangOpts.BuiltinHeadersInSystemModules &&
+         isBuiltinHeaderName(llvm::sys::path::filename(File->getName()));
 }
 
 ModuleMap::HeadersMap::iterator
@@ -1515,6 +1535,10 @@
     /// The active module.
     Module *ActiveModule = nullptr;
 
+    /// Whether parsed modules should be ignored (not counted in Map's NumCreatedModules
+    /// or added to Map's Modules/ModuleScopeIDs).
+    bool IgnoreModules = false;
+
     /// Whether a module uses the 'requires excluded' hack to mark its
     /// contents as 'textual'.
     ///
@@ -1947,6 +1971,7 @@
     return;
   }
 
+  bool SetIgnoreModules = false;
   if (ActiveModule) {
     if (Id.size() > 1) {
       Diags.Report(Id.front().second, diag::err_mmap_nested_submodule_id)
@@ -1955,12 +1980,21 @@
       HadError = true;
       return;
     }
-  } else if (Id.size() == 1 && Explicit) {
-    // Top-level modules can't be explicit.
-    Diags.Report(ExplicitLoc, diag::err_mmap_explicit_top_level);
-    Explicit = false;
-    ExplicitLoc = SourceLocation();
-    HadError = true;
+  } else if (Id.size() == 1) {
+    if (Explicit) {
+      // Top-level modules can't be explicit.
+      Diags.Report(ExplicitLoc, diag::err_mmap_explicit_top_level);
+      Explicit = false;
+      ExplicitLoc = SourceLocation();
+      HadError = true;
+    } else if (ModuleMapFile->getDir() == Map.BuiltinIncludeDir &&
+               Map.LangOpts.BuiltinHeadersInSystemModules &&
+               isBuiltInModuleName(Id[0].first)) {
+      // If the builtin headers belong to system modules, ignore
+      // their builtin modules.
+      IgnoreModules = true;
+      SetIgnoreModules = true;
+    }
   }
 
   Module *PreviousActiveModule = ActiveModule;
@@ -2078,8 +2112,12 @@
 
   // Start defining this module.
   if (ShadowingModule) {
+    assert(!IgnoreModules);
     ActiveModule =
         Map.createShadowedModule(ModuleName, Framework, ShadowingModule);
+  } else if (IgnoreModules) {
+    ActiveModule = new Module(ModuleName, SourceLocation(), ActiveModule,
+                              Framework, Explicit, 0);
   } else {
     ActiveModule =
         Map.findOrCreateModule(ModuleName, ActiveModule, Framework, Explicit)
@@ -2214,6 +2252,8 @@
 
   // We're done parsing this module. Pop back to the previous module.
   ActiveModule = PreviousActiveModule;
+  if (SetIgnoreModules)
+    IgnoreModules = false;
 }
 
 /// Parse an extern module declaration.
@@ -2249,20 +2289,22 @@
   std::string FileName = std::string(Tok.getString());
   consumeToken(); // filename
 
-  StringRef FileNameRef = FileName;
-  SmallString<128> ModuleMapFileName;
-  if (llvm::sys::path::is_relative(FileNameRef)) {
-    ModuleMapFileName += Directory.getName();
-    llvm::sys::path::append(ModuleMapFileName, FileName);
-    FileNameRef = ModuleMapFileName;
+  if (!IgnoreModules) {
+    StringRef FileNameRef = FileName;
+    SmallString<128> ModuleMapFileName;
+    if (llvm::sys::path::is_relative(FileNameRef)) {
+      ModuleMapFileName += Directory.getName();
+      llvm::sys::path::append(ModuleMapFileName, FileName);
+      FileNameRef = ModuleMapFileName;
+    }
+    if (auto File = SourceMgr.getFileManager().getOptionalFileRef(FileNameRef))
+      Map.parseModuleMapFile(
+          *File, IsSystem,
+          Map.HeaderInfo.getHeaderSearchOpts().ModuleMapFileHomeIsCwd
+              ? Directory
+              : File->getDir(),
+          FileID(), nullptr, ExternLoc);
   }
-  if (auto File = SourceMgr.getFileManager().getOptionalFileRef(FileNameRef))
-    Map.parseModuleMapFile(
-        *File, IsSystem,
-        Map.HeaderInfo.getHeaderSearchOpts().ModuleMapFileHomeIsCwd
-            ? Directory
-            : File->getDir(),
-        FileID(), nullptr, ExternLoc);
 }
 
 /// Whether to add the requirement \p Feature to the module \p M.
@@ -2472,7 +2514,8 @@
   }
 
   bool NeedsFramework = false;
-  Map.addUnresolvedHeader(ActiveModule, std::move(Header), NeedsFramework);
+  if (!IgnoreModules)
+    Map.addUnresolvedHeader(ActiveModule, std::move(Header), NeedsFramework);
 
   if (NeedsFramework)
     Diags.Report(CurrModuleDeclLoc, diag::note_mmap_add_framework_keyword)
@@ -2547,8 +2590,9 @@
     // Sort header paths so that the pcm doesn't depend on iteration order.
     llvm::array_pod_sort(Headers.begin(), Headers.end(), compareModuleHeaders);
 
-    for (auto &Header : Headers)
-      Map.addHeader(ActiveModule, std::move(Header), ModuleMap::TextualHeader);
+    if (!IgnoreModules)
+      for (auto &Header : Headers)
+        Map.addHeader(ActiveModule, std::move(Header), ModuleMap::TextualHeader);
     return;
   }
 
@@ -2559,8 +2603,9 @@
     return;
   }
 
-  // Record this umbrella directory.
-  Map.setUmbrellaDirAsWritten(ActiveModule, *Dir, DirNameAsWritten, DirName);
+  if (!IgnoreModules)
+    // Record this umbrella directory.
+    Map.setUmbrellaDirAsWritten(ActiveModule, *Dir, DirNameAsWritten, DirName);
 }
 
 /// Parse a module export declaration.
@@ -2643,7 +2688,8 @@
   }
 
   ActiveModule->ExportAsModule = std::string(Tok.getString());
-  Map.addLinkAsDependency(ActiveModule);
+  if (!IgnoreModules)
+    Map.addLinkAsDependency(ActiveModule);
 
   consumeToken();
 }
@@ -2871,6 +2917,7 @@
     ActiveModule->InferredSubmoduleLoc = StarLoc;
     ActiveModule->InferExplicitSubmodules = Explicit;
   } else {
+    assert(!IgnoreModules);
     // We'll be inferring framework modules for this directory.
     Map.InferredDirectories[Directory].InferModules = true;
     Map.InferredDirectories[Directory].Attrs = Attrs;
@@ -2910,8 +2957,9 @@
         break;
       }
 
-      Map.InferredDirectories[Directory].ExcludedModules.push_back(
-          std::string(Tok.getString()));
+      if (!IgnoreModules)
+        Map.InferredDirectories[Directory].ExcludedModules.push_back(
+            std::string(Tok.getString()));
       consumeToken();
       break;
 
diff --git a/clang/test/Driver/Inputs/MacOSX99.0.sdk/SDKSettings.json b/clang/test/Driver/Inputs/MacOSX99.0.sdk/SDKSettings.json
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/Inputs/MacOSX99.0.sdk/SDKSettings.json
@@ -0,0 +1 @@
+{"Version":"990.0", "MaximumDeploymentTarget": "99.0.99"}
diff --git a/clang/test/Driver/darwin-builtin-modules.c b/clang/test/Driver/darwin-builtin-modules.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/darwin-builtin-modules.c
@@ -0,0 +1,27 @@
+// Check that darwin passes -fbuiltin-headers-in-system-modules
+// when expected.
+
+// RUN: %clang -target x86_64-apple-darwin22.4 -### %s 2>&1 | FileCheck %s
+// RUN: %clang -isysroot %S/Inputs/MacOSX10.15.versioned.sdk -target x86_64-apple-macos10.15 -### %s 2>&1 | FileCheck %s
+// RUN: %clang -isysroot %S/Inputs/iPhoneOS13.0.sdk -target arm64-apple-ios13.0 -### %s 2>&1 | FileCheck %s
+// CHECK: -fbuiltin-headers-in-system-modules
+
+// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos98.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos99.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// CHECK_FUTURE-NOT: -fbuiltin-headers-in-system-modules
+
+
+// Check that builtin_headers_in_system_modules is only set if -fbuiltin-headers-in-system-modules and -fmodules are both set.
+
+// RUN: %clang -isysroot %S/Inputs/iPhoneOS13.0.sdk -target arm64-apple-ios13.0 -fsyntax-only %s -Xclang -verify=no-feature
+// RUN: %clang -isysroot %S/Inputs/iPhoneOS13.0.sdk -target arm64-apple-ios13.0 -fsyntax-only %s -fmodules -Xclang -verify=yes-feature
+// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos99.0 -fsyntax-only %s -Xclang -verify=no-feature
+// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos99.0 -fsyntax-only %s -fmodules -Xclang -verify=no-feature
+
+#if __has_feature(builtin_headers_in_system_modules)
+#error "has builtin_headers_in_system_modules"
+// yes-feature-error@-1 {{}}
+#else
+#error "no builtin_headers_in_system_modules"
+// no-feature-error@-1 {{}}
+#endif
diff --git a/clang/test/Modules/Werror-Wsystem-headers.m b/clang/test/Modules/Werror-Wsystem-headers.m
--- a/clang/test/Modules/Werror-Wsystem-headers.m
+++ b/clang/test/Modules/Werror-Wsystem-headers.m
@@ -3,19 +3,21 @@
 // RUN: mkdir %t-saved
 
 // Initial module build
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -fdisable-module-hash \
-// RUN:     -isystem %S/Inputs/System/usr/include -fsyntax-only %s -verify
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules \
+// RUN:     -fmodules-cache-path=%t -fdisable-module-hash -isystem %S/Inputs/System/usr/include \
+// RUN:     -fsyntax-only %s -verify
 // RUN: cp %t/cstd.pcm %t-saved/cstd.pcm
 
 // Even with -Werror don't rebuild a system module
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -fdisable-module-hash \
-// RUN:     -isystem %S/Inputs/System/usr/include -fsyntax-only %s -verify -Werror
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules \
+// RUN:     -fmodules-cache-path=%t -fdisable-module-hash -isystem %S/Inputs/System/usr/include \
+// RUN:     -fsyntax-only %s -verify -Werror
 // RUN: diff %t/cstd.pcm %t-saved/cstd.pcm
 
 // Unless -Wsystem-headers is on
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -fdisable-module-hash \
-// RUN:     -isystem %S/Inputs/System/usr/include -fsyntax-only %s -verify \
-// RUN:     -Werror=unused -Wsystem-headers
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules \
+// RUN:     -fmodules-cache-path=%t -fdisable-module-hash -isystem %S/Inputs/System/usr/include \
+// RUN:     -fsyntax-only %s -verify -Werror=unused -Wsystem-headers
 // RUN: not diff %t/cstd.pcm %t-saved/cstd.pcm
 
 // expected-no-diagnostics
diff --git a/clang/test/Modules/context-hash.c b/clang/test/Modules/context-hash.c
--- a/clang/test/Modules/context-hash.c
+++ b/clang/test/Modules/context-hash.c
@@ -4,22 +4,24 @@
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fsyntax-only -internal-isystem \
 // RUN:   %S/Inputs/System/usr/include -fmodules -fimplicit-module-maps \
-// RUN:   -fmodules-cache-path=%t %s -Rmodule-build 2> %t1
+// RUN:   -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t %s \
+// RUN:   -Rmodule-build 2> %t1
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fsyntax-only -internal-isystem \
 // RUN:   %S/Inputs/System/usr/include -internal-isystem %S -fmodules \
-// RUN:   -fimplicit-module-maps -fmodules-cache-path=%t %s -Rmodule-build 2> \
-// RUN:   %t2
+// RUN:   -fbuiltin-headers-in-system-modules -fimplicit-module-maps \
+// RUN:   -fmodules-cache-path=%t %s -Rmodule-build 2> %t2
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fsyntax-only -internal-isystem \
 // RUN:   %S/Inputs/System/usr/include -internal-isystem %S -fmodules \
-// RUN:   -fimplicit-module-maps -fmodules-cache-path=%t %s \
-// RUN:   -fmodules-strict-context-hash -Rmodule-build 2> %t3
+// RUN:   -fbuiltin-headers-in-system-modules -fimplicit-module-maps \
+// RUN:   -fmodules-cache-path=%t %s -fmodules-strict-context-hash \
+// RUN:   -Rmodule-build 2> %t3
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fsyntax-only -Weverything -internal-isystem \
 // RUN:   %S/Inputs/System/usr/include -fmodules -fmodules-strict-context-hash \
-// RUN:   -fimplicit-module-maps -fmodules-cache-path=%t %s -Rmodule-build 2> \
-// RUN:   %t4
+// RUN:   -fbuiltin-headers-in-system-modules -fimplicit-module-maps \
+// RUN:   -fmodules-cache-path=%t %s -Rmodule-build 2> %t4
 // RUN: echo %t > %t.path
 // RUN: cat %t.path %t1 %t2 %t3 %t4 | FileCheck %s
 
@@ -29,16 +31,17 @@
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fsyntax-only -internal-isystem \
 // RUN:   %S/Inputs/System/usr/include -fmodules -fimplicit-module-maps \
-// RUN:   -fmodules-cache-path=%t -x objective-c %s -Rmodule-build 2> %t1
+// RUN:   -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t \
+// RUN:   -x objective-c %s -Rmodule-build 2> %t1
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fsyntax-only -internal-isystem \
 // RUN:   %S/Inputs/System/usr/include -fmodules -fimplicit-module-maps \
-// RUN:   -fobjc-runtime=macosx-1.0.0.0 \
+// RUN:   -fbuiltin-headers-in-system-modules -fobjc-runtime=macosx-1.0.0.0 \
 // RUN:   -fmodules-cache-path=%t -x objective-c %s -Rmodule-build 2> %t2
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fsyntax-only -internal-isystem \
 // RUN:   %S/Inputs/System/usr/include -fmodules -fimplicit-module-maps \
-// RUN:   -fcomment-block-commands=lp,bj \
+// RUN:   -fbuiltin-headers-in-system-modules -fcomment-block-commands=lp,bj \
 // RUN:   -fmodules-cache-path=%t -x objective-c %s -Rmodule-build 2> %t3
 // RUN: echo %t > %t.path
 // RUN: cat %t.path %t1 %t2 %t3 | FileCheck --check-prefix=LANGOPTS %s
diff --git a/clang/test/Modules/crash-vfs-include-pch.m b/clang/test/Modules/crash-vfs-include-pch.m
--- a/clang/test/Modules/crash-vfs-include-pch.m
+++ b/clang/test/Modules/crash-vfs-include-pch.m
@@ -5,14 +5,14 @@
 
 // RUN: %clang_cc1 -x objective-c-header -emit-pch %S/Inputs/pch-used.h \
 // RUN:     -o %t/out/pch-used.h.pch -fmodules -fimplicit-module-maps \
-// RUN:     -fmodules-cache-path=%t/cache -O0 \
+// RUN:     -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t/cache -O0 \
 // RUN:     -isystem %S/Inputs/System/usr/include
 
 // RUN: env FORCE_CLANG_DIAGNOSTICS_CRASH= TMPDIR=%t TEMP=%t TMP=%t \
 // RUN: not %clang %s -E -include-pch %t/out/pch-used.h.pch -fmodules -nostdlibinc \
-// RUN:     -fimplicit-module-maps -fmodules-cache-path=%t/cache -O0 \
-// RUN:     -Xclang -fno-validate-pch -isystem %S/Inputs/System/usr/include \
-// RUN:     -o %t/output.E 2>&1 | FileCheck %s
+// RUN:     -fimplicit-module-maps -fbuiltin-headers-in-system-modules \
+// RUN:     -fmodules-cache-path=%t/cache -O0 -Xclang -fno-validate-pch \
+// RUN:     -isystem %S/Inputs/System/usr/include -o %t/output.E 2>&1 | FileCheck %s
 
 // RUN: FileCheck --check-prefix=CHECKSH %s -input-file %t/crash-vfs-*.sh
 // RUN: FileCheck --check-prefix=CHECKYAML %s -input-file \
diff --git a/clang/test/Modules/cstd.m b/clang/test/Modules/cstd.m
--- a/clang/test/Modules/cstd.m
+++ b/clang/test/Modules/cstd.m
@@ -1,5 +1,5 @@
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -fsyntax-only -internal-isystem %S/Inputs/System/usr/include -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -D__need_wint_t -Werror=implicit-function-declaration %s
+// RUN: %clang_cc1 -fsyntax-only -internal-isystem %S/Inputs/System/usr/include -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t -D__need_wint_t -Werror=implicit-function-declaration %s
 
 @import uses_other_constants;
 const double other_value = DBL_MAX;
diff --git a/clang/test/Modules/pch-used.m b/clang/test/Modules/pch-used.m
--- a/clang/test/Modules/pch-used.m
+++ b/clang/test/Modules/pch-used.m
@@ -1,8 +1,8 @@
 // UNSUPPORTED: target={{.*}}-zos{{.*}}, target={{.*}}-aix{{.*}}
 // RUN: rm -rf %t
 // RUN: mkdir %t
-// RUN: %clang_cc1 -x objective-c-header -emit-pch %S/Inputs/pch-used.h -o %t/pch-used.h.pch -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache -O0 -isystem %S/Inputs/System/usr/include
-// RUN: %clang_cc1 %s -include-pch %t/pch-used.h.pch -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache -O0 -isystem %S/Inputs/System/usr/include -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -x objective-c-header -emit-pch %S/Inputs/pch-used.h -o %t/pch-used.h.pch -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t/cache -O0 -isystem %S/Inputs/System/usr/include
+// RUN: %clang_cc1 %s -include-pch %t/pch-used.h.pch -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t/cache -O0 -isystem %S/Inputs/System/usr/include -emit-llvm -o - | FileCheck %s
 
 void f(void) { SPXTrace(); }
 void g(void) { double x = DBL_MAX; }
diff --git a/clang/test/Modules/shadowed-submodule.m b/clang/test/Modules/shadowed-submodule.m
--- a/clang/test/Modules/shadowed-submodule.m
+++ b/clang/test/Modules/shadowed-submodule.m
@@ -1,5 +1,5 @@
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/shadowed-submodule/Foo -I %S/Inputs/shadowed-submodule/A2 %s -verify
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t -I %S/Inputs/shadowed-submodule/Foo -I %S/Inputs/shadowed-submodule/A2 %s -verify
 
 @import Foo; // expected-error {{module 'A' was built in directory}}
              // expected-note@shadowed-submodule.m:4 {{imported by module 'Foo'}}
diff --git a/clang/test/Modules/stddef.c b/clang/test/Modules/stddef.c
--- a/clang/test/Modules/stddef.c
+++ b/clang/test/Modules/stddef.c
@@ -1,5 +1,5 @@
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I%S/Inputs/StdDef %s -verify -fno-modules-error-recovery
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t -I%S/Inputs/StdDef %s -verify -fno-modules-error-recovery
 
 #include "ptrdiff_t.h"
 
diff --git a/clang/test/Modules/stddef.m b/clang/test/Modules/stddef.m
--- a/clang/test/Modules/stddef.m
+++ b/clang/test/Modules/stddef.m
@@ -3,5 +3,5 @@
 size_t getSize(void);
 
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/StdDef %s -verify
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t -I %S/Inputs/StdDef %s -verify
 // expected-no-diagnostics