Add typing to update_checkout.py (swiftlang#81160)

As Python 3.5 added support for type hints and we require Python 3.6 or later, let's try to add a type hint to a single function so that development experience is slightly improved and we can test this feature in our code before applying it more widely. Editors with support for Python LSP servers can pick it and stop flagging this function or its
invocations as type errors.

Author: MaxDesiatov

utils/update_checkout/update_checkout/update_checkout.py

@@ -16,6 +16,7 @@
 import sys
 import traceback
 from multiprocessing import Lock, Pool, cpu_count, freeze_support
+from typing import Optional
 
 from build_swift.build_swift.constants import SWIFT_SOURCE_ROOT
 
@@ -74,7 +75,7 @@ def check_parallel_results(results, op):
     return fail_count
 
 
-def confirm_tag_in_repo(tag, repo_name):
+def confirm_tag_in_repo(tag, repo_name) -> Optional[str]:
     # type: (str, str) -> str | None
     """Confirm that a given tag exists in a git repository. This function
     assumes that the repository is already a current working directory before