[clang-tidy] Fix file extension inconsistency from #150791

In CheckRunner, we treat files with unrecognized extensions as ".cpp".
Make the new code be consistent with that.

Author: googlewalt

clang-tools-extra/test/clang-tidy/check_clang_tidy.py

@@ -392,7 +392,7 @@ def parse_arguments() -> Tuple[argparse.Namespace, List[str]]:
     if args.std is None:
         _, extension = os.path.splitext(args.assume_filename or args.input_file_name)
         args.std = [
-            "c++11-or-later" if extension in [".cpp", ".hpp", ".mm"] else "c99-or-later"
+            "c99-or-later" if extension in [".c", ".m"] else "c++11-or-later"
         ]
 
     return (args, extra_args)