undef reviewer: tweak the regex so it doesn't warn about #undef

Author: nunoplopes

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

@@ -380,7 +380,7 @@ def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str
         # diff --git a/file b/file
         for file in re.split("^diff --git ", stdout, 0, re.MULTILINE):
             # search for additions of undef
-            if re.search("^[+].*" + regex, file, re.MULTILINE):
+            if re.search(r"^[+](?!\s*#\s*).*(\bundef\b|UndefValue::get)", file, re.MULTILINE):
                 files.append(re.match("a/([^ ]+)", file.splitlines()[0])[1])
 
         if not files: