[update_cc_test_checks.py] Use CHECK_RE from common
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
Differential Revision: https://reviews.llvm.org/D70890