[update_cc_test_checks.py] Use CHECK_RE from common

Summary:
This change modifies the common.CHECK_RE regex to also handle '//'
comment prefixes which allows us to share it between clang and IR tests.
Using the regex from common means that *-SAME lines are also stripped
now. Before this change using the --function-signature flag would result
in -SAME: lines from previous runs not being removed.

Reviewers: MaskRay, jdoerfert

Reviewed By: jdoerfert

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70890