diff --git a/clang-tools-extra/clangd/test/lit.cfg.py b/clang-tools-extra/clangd/test/lit.cfg.py
--- a/clang-tools-extra/clangd/test/lit.cfg.py
+++ b/clang-tools-extra/clangd/test/lit.cfg.py
@@ -36,3 +36,8 @@
 
 if config.have_zlib:
   config.available_features.add('zlib')
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang-tools-extra/clangd/unittests/lit.cfg.py b/clang-tools-extra/clangd/unittests/lit.cfg.py
--- a/clang-tools-extra/clangd/unittests/lit.cfg.py
+++ b/clang-tools-extra/clangd/unittests/lit.cfg.py
@@ -17,5 +17,7 @@
     "@SHLIBDIR@", "@LLVM_LIBS_DIR@",
     config.environment.get(shlibpath_var,'')))
 
-
-
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang-tools-extra/include-cleaner/test/Unit/lit.cfg.py b/clang-tools-extra/include-cleaner/test/Unit/lit.cfg.py
--- a/clang-tools-extra/include-cleaner/test/Unit/lit.cfg.py
+++ b/clang-tools-extra/include-cleaner/test/Unit/lit.cfg.py
@@ -16,3 +16,8 @@
 config.environment[shlibpath_var] = os.path.pathsep.join((
     "@SHLIBDIR@", "@LLVM_LIBS_DIR@",
     config.environment.get(shlibpath_var,'')))
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang-tools-extra/include-cleaner/test/lit.cfg.py b/clang-tools-extra/include-cleaner/test/lit.cfg.py
--- a/clang-tools-extra/include-cleaner/test/lit.cfg.py
+++ b/clang-tools-extra/include-cleaner/test/lit.cfg.py
@@ -14,3 +14,8 @@
         config.clang_tools_dir,
         config.llvm_tools_dir,
         config.environment['PATH']))
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang-tools-extra/pseudo/test/Unit/lit.cfg.py b/clang-tools-extra/pseudo/test/Unit/lit.cfg.py
--- a/clang-tools-extra/pseudo/test/Unit/lit.cfg.py
+++ b/clang-tools-extra/pseudo/test/Unit/lit.cfg.py
@@ -17,3 +17,7 @@
     "@SHLIBDIR@", "@LLVM_LIBS_DIR@",
     config.environment.get(shlibpath_var,'')))
 
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang-tools-extra/pseudo/test/lit.cfg.py b/clang-tools-extra/pseudo/test/lit.cfg.py
--- a/clang-tools-extra/pseudo/test/lit.cfg.py
+++ b/clang-tools-extra/pseudo/test/lit.cfg.py
@@ -14,3 +14,8 @@
         config.clang_tools_dir,
         config.llvm_tools_dir,
         config.environment['PATH']))
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang-tools-extra/test/Unit/lit.cfg.py b/clang-tools-extra/test/Unit/lit.cfg.py
--- a/clang-tools-extra/test/Unit/lit.cfg.py
+++ b/clang-tools-extra/test/Unit/lit.cfg.py
@@ -35,3 +35,8 @@
     shlibpath = os.path.pathsep.join((config.shlibdir, shlibpath))
 
 config.environment[shlibpath_var] = shlibpath
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang-tools-extra/test/lit.cfg.py b/clang-tools-extra/test/lit.cfg.py
--- a/clang-tools-extra/test/lit.cfg.py
+++ b/clang-tools-extra/test/lit.cfg.py
@@ -59,3 +59,8 @@
 # Plugins (loadable modules)
 if config.has_plugins and config.llvm_plugin_ext:
     config.available_features.add('plugins')
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"