diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1423,16 +1423,10 @@
 # path. Since this uses __file__, it has to be emitted into python files that
 # use it and can't be in a lit module. Use with make_paths_relative().
 string(CONCAT LLVM_LIT_PATH_FUNCTION
-  # Lit converts config paths to lower case in discovery.py, before
-  # loading the config. This causes __file__ to be all lower-case (including
-  # the drive letter), but several clang tests pass -include %s and a
-  # clang warning checks that passed case matches on-disk cache. So it's
-  # important that this restores the on-disk case of the prefix.
   "# Allow generated file to be relocatable.\n"
   "def path(p):\n"
   "    if not p: return ''\n"
   "    p = os.path.join(os.path.dirname(os.path.abspath(__file__)), p)\n"
-  "    if os.name == 'nt' and os.path.isabs(p): return p[0].upper() + p[1:]\n"
   "    return p\n"
   )
 
diff --git a/llvm/utils/lit/lit/discovery.py b/llvm/utils/lit/lit/discovery.py
--- a/llvm/utils/lit/lit/discovery.py
+++ b/llvm/utils/lit/lit/discovery.py
@@ -53,8 +53,8 @@
         config_map = litConfig.params.get('config_map')
         if config_map:
             cfgpath = os.path.realpath(cfgpath)
-            cfgpath = os.path.normcase(cfgpath)
-            target = config_map.get(cfgpath)
+            t, target = config_map.get(os.path.normcase(cfgpath), (None, None))
+            assert t is None or t == cfgpath
             if target:
                 cfgpath = target
 
diff --git a/llvm/utils/lit/lit/llvm/config.py b/llvm/utils/lit/lit/llvm/config.py
--- a/llvm/utils/lit/lit/llvm/config.py
+++ b/llvm/utils/lit/lit/llvm/config.py
@@ -115,8 +115,8 @@
 
     def with_environment(self, variable, value, append_path=False):
         if append_path:
-            # For paths, we should be able to take a list of them and process all
-            # of them.
+            # For paths, we should be able to take a list of them and process
+            # all of them.
             paths_to_add = value
             if lit.util.is_string(paths_to_add):
                 paths_to_add = [paths_to_add]
@@ -135,8 +135,8 @@
             # and adding each to the beginning would result in b c a.  So we
             # need to iterate in reverse to end up with the original ordering.
             for p in reversed(paths_to_add):
-                # Move it to the front if it already exists, otherwise insert it at the
-                # beginning.
+                # Move it to the front if it already exists, otherwise insert
+                # it at the beginning.
                 p = norm(p)
                 try:
                     paths.remove(p)
diff --git a/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py b/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py
--- a/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py
+++ b/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py
@@ -6,7 +6,7 @@
 main_config = os.path.realpath(main_config)
 main_config = os.path.normcase(main_config)
 
-config_map = {main_config : sys.argv[2]}
+config_map = {os.path.normcase(main_config) : (main_config, sys.argv[2])}
 builtin_parameters = {'config_map' : config_map}
 
 if __name__=='__main__':
diff --git a/llvm/utils/llvm-lit/llvm-lit.in b/llvm/utils/llvm-lit/llvm-lit.in
--- a/llvm/utils/llvm-lit/llvm-lit.in
+++ b/llvm/utils/llvm-lit/llvm-lit.in
@@ -9,9 +9,8 @@
 def map_config(source_dir, site_config):
     global config_map
     source_dir = os.path.realpath(source_dir)
-    source_dir = os.path.normcase(source_dir)
     site_config = os.path.normpath(site_config)
-    config_map[source_dir] = site_config
+    config_map[os.path.normcase(source_dir)] = source_dir, site_config
 
 # Set up some builtin parameters, so that by default the LLVM test suite
 # configuration file knows how to find the object tree.