Try again to get tests passing again on Windows.

Things pass locally, but some tests on some bots are still unhappy.
I'm not sure why. See if using forward slashes as before helps.
diff --git a/clang/test/Unit/lit.site.cfg.py.in b/clang/test/Unit/lit.site.cfg.py.in
index ad5e6d0..400a9c0 100644
--- a/clang/test/Unit/lit.site.cfg.py.in
+++ b/clang/test/Unit/lit.site.cfg.py.in
@@ -2,14 +2,14 @@
 
 import sys
 
-config.llvm_src_root = path(r"@LLVM_SOURCE_DIR@")
-config.llvm_obj_root = path(r"@LLVM_BINARY_DIR@")
-config.llvm_tools_dir = path(r"@LLVM_TOOLS_DIR@")
-config.llvm_libs_dir = path(r"@LLVM_LIBS_DIR@")
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
+config.llvm_libs_dir = path("@LLVM_LIBS_DIR@")
 config.llvm_build_mode = "@LLVM_BUILD_MODE@"
-config.clang_obj_root = path(r"@CLANG_BINARY_DIR@")
+config.clang_obj_root = path("@CLANG_BINARY_DIR@")
 config.enable_shared = @ENABLE_SHARED@
-config.shlibdir = path(r"@SHLIBDIR@")
+config.shlibdir = path("@SHLIBDIR@")
 config.target_triple = "@TARGET_TRIPLE@"
 
 # Support substitution of the tools_dir, libs_dirs, and build_mode with user
@@ -26,4 +26,4 @@
 
 # Let the main config do the real work.
 lit_config.load_config(
-    config, os.path.join(path(r"@CLANG_SOURCE_DIR@"), "test/Unit/lit.cfg.py"))
+    config, os.path.join(path("@CLANG_SOURCE_DIR@"), "test/Unit/lit.cfg.py"))
diff --git a/clang/test/lit.site.cfg.py.in b/clang/test/lit.site.cfg.py.in
index daec694..84dd0f8 100644
--- a/clang/test/lit.site.cfg.py.in
+++ b/clang/test/lit.site.cfg.py.in
@@ -2,16 +2,16 @@
 
 import sys
 
-config.llvm_src_root = path(r"@LLVM_SOURCE_DIR@")
-config.llvm_obj_root = path(r"@LLVM_BINARY_DIR@")
-config.llvm_tools_dir = path(r"@LLVM_TOOLS_DIR@")
-config.llvm_libs_dir = path(r"@LLVM_LIBS_DIR@")
-config.llvm_shlib_dir = path(r"@SHLIBDIR@")
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
+config.llvm_libs_dir = path("@LLVM_LIBS_DIR@")
+config.llvm_shlib_dir = path("@SHLIBDIR@")
 config.llvm_plugin_ext = "@LLVM_PLUGIN_EXT@"
-config.lit_tools_dir = path(r"@LLVM_LIT_TOOLS_DIR@")
-config.clang_obj_root = path(r"@CLANG_BINARY_DIR@")
-config.clang_src_dir = path(r"@CLANG_SOURCE_DIR@")
-config.clang_tools_dir = path(r"@CLANG_TOOLS_DIR@")
+config.lit_tools_dir = path("@LLVM_LIT_TOOLS_DIR@")
+config.clang_obj_root = path("@CLANG_BINARY_DIR@")
+config.clang_src_dir = path("@CLANG_SOURCE_DIR@")
+config.clang_tools_dir = path("@CLANG_TOOLS_DIR@")
 config.host_triple = "@LLVM_HOST_TRIPLE@"
 config.target_triple = "@TARGET_TRIPLE@"
 config.host_cxx = "@CMAKE_CXX_COMPILER@"
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index 0732ca6..a474071 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1486,7 +1486,7 @@
     string(REPLACE ";" "\\;" ARG_PATH_VALUES_ESCAPED "${ARG_PATH_VALUES}")
     get_filename_component(OUTPUT_DIR ${site_out} DIRECTORY)
     execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c"
-      "import os, sys; sys.stdout.write(';'.join(os.path.relpath(p, sys.argv[1]) if p else '' for p in sys.argv[2].split(';')))"
+      "import os, sys; sys.stdout.write(';'.join(os.path.relpath(p, sys.argv[1]).replace('\\\\', '/') if p else '' for p in sys.argv[2].split(';')))"
       ${OUTPUT_DIR}
       ${ARG_PATH_VALUES_ESCAPED}
       OUTPUT_VARIABLE ARG_PATH_VALUES_RELATIVE)
diff --git a/llvm/test/Unit/lit.site.cfg.py.in b/llvm/test/Unit/lit.site.cfg.py.in
index f9fe421..3358ccf 100644
--- a/llvm/test/Unit/lit.site.cfg.py.in
+++ b/llvm/test/Unit/lit.site.cfg.py.in
@@ -2,12 +2,12 @@
 
 import sys
 
-config.llvm_src_root = path(r"@LLVM_SOURCE_DIR@")
-config.llvm_obj_root = path(r"@LLVM_BINARY_DIR@")
-config.llvm_tools_dir = path(r"@LLVM_TOOLS_DIR@")
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
 config.llvm_build_mode = "@LLVM_BUILD_MODE@"
 config.enable_shared = @ENABLE_SHARED@
-config.shlibdir = path(r"@SHLIBDIR@")
+config.shlibdir = path("@SHLIBDIR@")
 
 # Support substitution of the tools_dir and build_mode with user parameters.
 # This is used when we can't determine the tool dir at configuration time.
diff --git a/llvm/test/lit.site.cfg.py.in b/llvm/test/lit.site.cfg.py.in
index ee9e594..2af0ec3 100644
--- a/llvm/test/lit.site.cfg.py.in
+++ b/llvm/test/lit.site.cfg.py.in
@@ -4,14 +4,14 @@
 
 config.host_triple = "@LLVM_HOST_TRIPLE@"
 config.target_triple = "@TARGET_TRIPLE@"
-config.llvm_src_root = path(r"@LLVM_SOURCE_DIR@")
-config.llvm_obj_root = path(r"@LLVM_BINARY_DIR@")
-config.llvm_tools_dir = path(r"@LLVM_TOOLS_DIR@")
-config.llvm_lib_dir = path(r"@LLVM_LIBRARY_DIR@")
-config.llvm_shlib_dir = path(r"@SHLIBDIR@")
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
+config.llvm_lib_dir = path("@LLVM_LIBRARY_DIR@")
+config.llvm_shlib_dir = path("@SHLIBDIR@")
 config.llvm_shlib_ext = "@SHLIBEXT@"
 config.llvm_exe_ext = "@EXEEXT@"
-config.lit_tools_dir = path(r"@LLVM_LIT_TOOLS_DIR@")
+config.lit_tools_dir = path("@LLVM_LIT_TOOLS_DIR@")
 config.python_executable = "@PYTHON_EXECUTABLE@"
 config.gold_executable = "@GOLD_EXECUTABLE@"
 config.ld64_executable = "@LD64_EXECUTABLE@"