[CI][format] Explicitly pass extensions to git-clang-format (llvm#95794)

This ensures that the CI script controls which file extensions are
considered instead of letting git-clang-format apply its own filtering
rules. In particular, this properly handles libc++ extension-less
headers which were passed to git-clang-format, but then dropped by that
tool as having an unrecognized extension.

Author: ldionne

llvm/utils/git/code-format-helper.py

@@ -216,6 +216,17 @@ def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str
             cf_cmd.append(args.start_rev)
             cf_cmd.append(args.end_rev)
 
+        # Gather the extension of all modified files and pass them explicitly to git-clang-format.
+        # This prevents git-clang-format from applying its own filtering rules on top of ours.
+        extensions = set()
+        for file in cpp_files:
+            _, ext = os.path.splitext(file)
+            extensions.add(
+                ext.strip(".")
+            )  # Exclude periods since git-clang-format takes extensions without them
+        cf_cmd.append("--extensions")
+        cf_cmd.append("'{}'".format(",".join(extensions)))
+
         cf_cmd.append("--")
         cf_cmd += cpp_files