[CI][format] Explicitly pass extensions to git-clang-format (take 2) (#98227)

This patch 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 
the tool as having an unrecognized extension.

This is a second attempt to land 7620fe0, which was reverted in
9572388 because it caused formatting not to be enforced for several patches.
The problem was that we'd incorrectly pass the extensions with additional
quoting to git-clang-format. The incorrect quoting has been removed in this
version of the patch.

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(",".join(extensions))
+
         cf_cmd.append("--")
         cf_cmd += cpp_files