diff --git a/clang-tools-extra/clangd/test/lit.site.cfg.py.in b/clang-tools-extra/clangd/test/lit.site.cfg.py.in
--- a/clang-tools-extra/clangd/test/lit.site.cfg.py.in
+++ b/clang-tools-extra/clangd/test/lit.site.cfg.py.in
@@ -2,24 +2,15 @@
 
 # Variables needed for common clang config.
 config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
-config.clang_tools_dir = "@CLANG_TOOLS_DIR@"
-config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
 config.clang_libs_dir = "@CLANG_LIBS_DIR@"
-config.llvm_libs_dir = "@LLVM_LIBS_DIR@"
 config.target_triple = "@TARGET_TRIPLE@"
 config.host_triple = "@LLVM_HOST_TRIPLE@"
 config.python_executable = "@Python3_EXECUTABLE@"
-
 # Support substitution of the tools and libs dirs with user parameters. This is
 # used when we can't determine the tool dir at configuration time.
-try:
-    config.clang_tools_dir = config.clang_tools_dir % lit_config.params
-    config.llvm_tools_dir = config.llvm_tools_dir % lit_config.params
-    config.llvm_libs_dir = config.llvm_libs_dir % lit_config.params
-except KeyError:
-    e = sys.exc_info()[1]
-    key, = e.args
-    lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
+config.clang_tools_dir = lit_config.substitute("@CLANG_TOOLS_DIR@")
+config.llvm_tools_dir = lit_config.substitute("@LLVM_TOOLS_DIR@")
+config.llvm_libs_dir = lit_config.substitute("@LLVM_LIBS_DIR@")
 
 config.clangd_source_dir = "@CMAKE_CURRENT_SOURCE_DIR@/.."
 config.clangd_binary_dir = "@CMAKE_CURRENT_BINARY_DIR@/.."
diff --git a/clang-tools-extra/test/lit.site.cfg.py.in b/clang-tools-extra/test/lit.site.cfg.py.in
--- a/clang-tools-extra/test/lit.site.cfg.py.in
+++ b/clang-tools-extra/test/lit.site.cfg.py.in
@@ -2,28 +2,19 @@
 
 import sys
 
-config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
-config.llvm_libs_dir = "@LLVM_LIBS_DIR@"
 config.llvm_plugin_ext = "@LLVM_PLUGIN_EXT@"
 config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
 config.clang_tools_binary_dir = "@CLANG_TOOLS_BINARY_DIR@"
-config.clang_tools_dir = "@CLANG_TOOLS_DIR@"
 config.clang_libs_dir = "@SHLIBDIR@"
 config.python_executable = "@Python3_EXECUTABLE@"
 config.target_triple = "@TARGET_TRIPLE@"
 config.clang_tidy_staticanalyzer = @CLANG_TIDY_ENABLE_STATIC_ANALYZER@
 config.has_plugins = @CLANG_PLUGIN_SUPPORT@ & ~@LLVM_INSTALL_TOOLCHAIN_ONLY@
-
 # Support substitution of the tools and libs dirs with user parameters. This is
 # used when we can't determine the tool dir at configuration time.
-try:
-    config.clang_tools_dir = config.clang_tools_dir % lit_config.params
-    config.llvm_tools_dir = config.llvm_tools_dir % lit_config.params
-    config.llvm_libs_dir = config.llvm_libs_dir % lit_config.params
-except KeyError:
-    e = sys.exc_info()[1]
-    key, = e.args
-    lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
+config.llvm_tools_dir = lit_config.substitute("@LLVM_TOOLS_DIR@")
+config.llvm_libs_dir = lit_config.substitute("@LLVM_LIBS_DIR@")
+config.clang_tools_dir = lit_config.substitute("@CLANG_TOOLS_DIR@")
 
 import lit.llvm
 lit.llvm.initialize(lit_config, config)
diff --git a/llvm/utils/lit/lit/LitConfig.py b/llvm/utils/lit/lit/LitConfig.py
--- a/llvm/utils/lit/lit/LitConfig.py
+++ b/llvm/utils/lit/lit/LitConfig.py
@@ -175,6 +175,15 @@
             # output.
             sys.stderr.flush()
 
+    def substitute(self, string):
+        """substitute - Interpolate params into a string"""
+        try:
+          return string % self.params
+        except KeyError as e:
+          key, = e.args
+          self.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (
+              key,key))
+
     def note(self, message):
         if not self.quiet:
             self._write_message('note', message)