Index: lldb/packages/Python/lldbsuite/test/builders/builder.py
===================================================================
--- lldb/packages/Python/lldbsuite/test/builders/builder.py
+++ lldb/packages/Python/lldbsuite/test/builders/builder.py
@@ -120,6 +120,11 @@
                 configuration.clang_module_cache_dir)]
         return []
 
+    def getLibCxxArgs(self):
+        if configuration.hermetic_libcxx:
+            return ["USE_HERMETIC_LIBCPP=1"]
+        return []
+
     def _getDebugInfoArgs(self, debug_info):
         if debug_info is None:
             return []
@@ -142,7 +147,7 @@
             self.getArchCFlags(architecture), self.getArchSpec(architecture),
             self.getCCSpec(compiler), self.getExtraMakeArgs(),
             self.getSDKRootSpec(), self.getModuleCacheSpec(),
-            self.getCmdLine(dictionary)]
+            self.getLibCxxArgs(), self.getCmdLine(dictionary)]
         command = list(itertools.chain(*command_parts))
 
         return command
Index: lldb/packages/Python/lldbsuite/test/configuration.py
===================================================================
--- lldb/packages/Python/lldbsuite/test/configuration.py
+++ lldb/packages/Python/lldbsuite/test/configuration.py
@@ -124,6 +124,9 @@
 # LLDB library directory.
 lldb_libs_dir = None
 
+# Force us to use the just-built libcxx
+hermetic_libcxx = False
+
 # A plugin whose tests will be enabled, like intel-pt.
 enabled_plugins = []
 
Index: lldb/packages/Python/lldbsuite/test/dotest.py
===================================================================
--- lldb/packages/Python/lldbsuite/test/dotest.py
+++ lldb/packages/Python/lldbsuite/test/dotest.py
@@ -281,6 +281,8 @@
         logging.warning('No valid FileCheck executable; some tests may fail...')
         logging.warning('(Double-check the --llvm-tools-dir argument to dotest.py)')
 
+    configuration.hermetic_libxxx = args.hermetic_libcxx
+
     if args.channels:
         lldbtest_config.channels = args.channels
 
Index: lldb/packages/Python/lldbsuite/test/dotest_args.py
===================================================================
--- lldb/packages/Python/lldbsuite/test/dotest_args.py
+++ lldb/packages/Python/lldbsuite/test/dotest_args.py
@@ -43,6 +43,8 @@
     if sys.platform == 'darwin':
         group.add_argument('--apple-sdk', metavar='apple_sdk', dest='apple_sdk', default="", help=textwrap.dedent(
             '''Specify the name of the Apple SDK (macosx, macosx.internal, iphoneos, iphoneos.internal, or path to SDK) and use the appropriate tools from that SDK's toolchain.'''))
+    group.add_argument('--hermetic-libcxx', action='store_true', help=textwrap.dedent(
+        '''Force the just-built libcxx to be used for the libc++ formatter tests.'''))
     # FIXME? This won't work for different extra flags according to each arch.
     group.add_argument(
         '-E',
Index: lldb/packages/Python/lldbsuite/test/make/Makefile.rules
===================================================================
--- lldb/packages/Python/lldbsuite/test/make/Makefile.rules
+++ lldb/packages/Python/lldbsuite/test/make/Makefile.rules
@@ -387,17 +387,22 @@
 endif
 
 ifeq (1,$(USE_LIBCPP))
-	CXXFLAGS += -DLLDB_USING_LIBCPP
-	ifeq "$(OS)" "Android"
-		# Nothing to do, this is already handled in
-		# Android.rules.
+	ifeq (1,$(USE_HERMETIC_LIBCPP))
+		CXXFLAGS += -nostdlib++ -nostdinc++ -I$(LLVM_INCLUDE_DIR)/c++/v1
+		LDFLAGS += -L$(LLVM_LIBS_DIR) -Wl,-rpath,$(LLVM_LIBS_DIR) -lc++
 	else
-		CXXFLAGS += -stdlib=libc++
-		LDFLAGS += -stdlib=libc++
-	endif
-	ifneq (,$(filter $(OS), FreeBSD Linux NetBSD))
-		ifneq (,$(LLVM_LIBS_DIR))
-			LDFLAGS += -Wl,-rpath,$(LLVM_LIBS_DIR)
+		CXXFLAGS += -DLLDB_USING_LIBCPP
+		ifeq "$(OS)" "Android"
+			# Nothing to do, this is already handled in
+			# Android.rules.
+		else
+			CXXFLAGS += -stdlib=libc++
+			LDFLAGS += -stdlib=libc++
+		endif
+		ifneq (,$(filter $(OS), FreeBSD Linux NetBSD))
+			ifneq (,$(LLVM_LIBS_DIR))
+				LDFLAGS += -Wl,-rpath,$(LLVM_LIBS_DIR)
+			endif
 		endif
 	endif
 endif
Index: lldb/test/API/lit.cfg.py
===================================================================
--- lldb/test/API/lit.cfg.py
+++ lldb/test/API/lit.cfg.py
@@ -158,14 +158,22 @@
 if is_configured('dotest_args_str'):
   dotest_cmd.extend(config.dotest_args_str.split(';'))
 
-# Library path may be needed to locate just-built clang.
+# Library path may be needed to locate just-built clang and libcxx.
 if is_configured('llvm_libs_dir'):
   dotest_cmd += ['--env', 'LLVM_LIBS_DIR=' + config.llvm_libs_dir]
 
+# Include path may be needed to locate just-built libcxx.
+if is_configured('llvm_include_dir'):
+  dotest_cmd += ['--env', 'LLVM_INCLUDE_DIR=' + config.llvm_include_dir]
+
 # This path may be needed to locate required llvm tools
 if is_configured('llvm_tools_dir'):
   dotest_cmd += ['--env', 'LLVM_TOOLS_DIR=' + config.llvm_tools_dir]
 
+# If we have a just-built libcxx, prefer it over the system one.
+if is_configured('has_libcxx'):
+  dotest_cmd += ['--hermetic-libcxx']
+
 # Forward ASan-specific environment variables to tests, as a test may load an
 # ASan-ified dylib.
 for env_var in ('ASAN_OPTIONS', 'DYLD_INSERT_LIBRARIES'):
Index: lldb/test/API/lit.site.cfg.py.in
===================================================================
--- lldb/test/API/lit.site.cfg.py.in
+++ lldb/test/API/lit.site.cfg.py.in
@@ -4,6 +4,7 @@
 config.llvm_obj_root = "@LLVM_BINARY_DIR@"
 config.llvm_tools_dir = lit_config.substitute("@LLVM_TOOLS_DIR@")
 config.llvm_libs_dir = lit_config.substitute("@LLVM_LIBS_DIR@")
+config.llvm_include_dir = lit_config.substitute("@LLVM_INCLUDE_DIR@")
 config.llvm_shlib_dir = lit_config.substitute("@SHLIBDIR@")
 config.llvm_build_mode = lit_config.substitute("@LLVM_BUILD_MODE@")
 config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
@@ -29,6 +30,7 @@
 config.test_arch = '@LLDB_TEST_ARCH@'
 config.test_compiler = lit_config.substitute('@LLDB_TEST_COMPILER@')
 config.dsymutil = lit_config.substitute('@LLDB_TEST_DSYMUTIL@')
+config.has_libcxx = '@LLDB_HAS_LIBCXX@'
 # The API tests use their own module caches.
 config.lldb_module_cache = os.path.join("@LLDB_TEST_MODULE_CACHE_LLDB@", "lldb-api")
 config.clang_module_cache = os.path.join("@LLDB_TEST_MODULE_CACHE_CLANG@", "lldb-api")
Index: lldb/test/CMakeLists.txt
===================================================================
--- lldb/test/CMakeLists.txt
+++ lldb/test/CMakeLists.txt
@@ -127,6 +127,7 @@
   # dependency as it's also possible to run the libc++ tests against the libc++
   # installed on the system.
   if (TARGET cxx)
+    set(LLDB_HAS_LIBCXX ON)
     add_lldb_test_dependency(cxx)
   endif()
 
@@ -172,6 +173,7 @@
   LLDB_ENABLE_LZMA
   LLVM_ENABLE_ZLIB
   LLVM_ENABLE_SHARED_LIBS
+  LLDB_HAS_LIBCXX
   LLDB_USE_SYSTEM_DEBUGSERVER
   LLDB_IS_64_BITS)