[update_cc_test_checks.py] Add the --function-signature flag

Summary:
This was added to update_test_checks.py in D68819 and I believe having it
in update_cc_test_checks.py is also useful.

Reviewers: jdoerfert, MaskRay

Reviewed By: MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70429