Index: clang/docs/UsersManual.rst
===================================================================
--- clang/docs/UsersManual.rst
+++ clang/docs/UsersManual.rst
@@ -3134,7 +3134,7 @@
       --version               Print version information
       -v                      Show commands to run and use verbose output
       -W<warning>             Enable the specified warning
-      -Xclang <arg>           Pass <arg> to the clang compiler
+      -Xclang <arg>           Pass <arg> to the clang cc1 frontend. For experimental use only.
 
 The /clang: Option
 ^^^^^^^^^^^^^^^^^^
Index: clang/include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -405,4 +405,14 @@
 def warn_drv_moutline_unsupported_opt : Warning<
   "The '%0' architecture does not support -moutline; flag ignored">,
   InGroup<OptionIgnored>;
+
+def warn_drv_xclang_option : Warning<
+  "-Xclang options are for experimental use only; future compatibility may not be preserved">,
+  InGroup<ExperimentalDriverOption>, DefaultWarnNoWerror;
+
+def warn_drv_mllvm_option : Warning<
+  "-mllvm options are for experimental use only; future compatibility may not be preserved">,
+  InGroup<ExperimentalDriverOption>, DefaultWarnNoWerror;
+
 }
+
Index: clang/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- clang/include/clang/Basic/DiagnosticGroups.td
+++ clang/include/clang/Basic/DiagnosticGroups.td
@@ -1039,3 +1039,7 @@
 // A warning group specifically for warnings related to function
 // multiversioning.
 def FunctionMultiVersioning : DiagGroup<"function-multiversion">;
+
+// Warning for command-line options that are not a supported part of the clang command-line interface.
+// Warnings in this group should be on by default, and exempt from -Werror.
+def ExperimentalDriverOption : DiagGroup<"experimental-driver-option">;
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -466,7 +466,7 @@
   HelpText<"Pass <arg> to the assembler">, MetaVarName<"<arg>">,
   Group<CompileOnly_Group>;
 def Xclang : Separate<["-"], "Xclang">,
-  HelpText<"Pass <arg> to the clang compiler">, MetaVarName<"<arg>">,
+  HelpText<"Pass <arg> to the clang cc1 frontend. For experimental use only">, MetaVarName<"<arg>">,
   Flags<[DriverOption, CoreOption]>, Group<CompileOnly_Group>;
 def Xcuda_fatbinary : Separate<["-"], "Xcuda-fatbinary">,
   HelpText<"Pass <arg> to fatbinary invocation">, MetaVarName<"<arg>">;
@@ -2002,7 +2002,7 @@
 def mlinker_version_EQ : Joined<["-"], "mlinker-version=">,
   Flags<[DriverOption]>;
 def mllvm : Separate<["-"], "mllvm">, Flags<[CC1Option,CC1AsOption,CoreOption]>,
-  HelpText<"Additional arguments to forward to LLVM's option processing">;
+  HelpText<"Additional arguments to forward to LLVM's option processing. For experimental use only">;
 def mmacosx_version_min_EQ : Joined<["-"], "mmacosx-version-min=">,
   Group<m_Group>, HelpText<"Set Mac OS X deployment target">;
 def mmacos_version_min_EQ : Joined<["-"], "mmacos-version-min=">,
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -4994,6 +4994,11 @@
 
   // Forward -Xclang arguments to -cc1, and -mllvm arguments to the LLVM option
   // parser.
+  if (Args.hasArg(options::OPT_Xclang))
+    D.Diag(diag::warn_drv_xclang_option);
+  if (Args.hasArg(options::OPT_mllvm))
+    D.Diag(diag::warn_drv_mllvm_option);
+
   // -finclude-default-header flag is for preprocessor,
   // do not pass it to other cc1 commands when save-temps is enabled
   if (C.getDriver().isSaveTempsEnabled() &&
@@ -5926,6 +5931,8 @@
   CollectArgsForIntegratedAssembler(C, Args, CmdArgs,
                                     getToolChain().getDriver());
 
+  if (Args.hasArg(options::OPT_mllvm))
+    D.Diag(diag::warn_drv_mllvm_option);
   Args.AddAllArgs(CmdArgs, options::OPT_mllvm);
 
   assert(Output.isFilename() && "Unexpected lipo output.");
Index: clang/tools/libclang/CIndex.cpp
===================================================================
--- clang/tools/libclang/CIndex.cpp
+++ clang/tools/libclang/CIndex.cpp
@@ -3419,6 +3419,11 @@
   if (options & CXTranslationUnit_KeepGoing)
     Diags->setSuppressAfterFatalError(false);
 
+  // Suppress driver warning for use of -Xclang, since we add it ourselves.
+  Diags->setSeverityForGroup(diag::Flavor::WarningOrError,
+                             "experimental-driver-option",
+                             diag::Severity::Ignored, SourceLocation());
+
   // Recover resources if we crash before exiting this function.
   llvm::CrashRecoveryContextCleanupRegistrar<DiagnosticsEngine,
     llvm::CrashRecoveryContextReleaseRefCleanup<DiagnosticsEngine> >