[UpdateTestChecks] Add UTC_ARGS support for update_{llc,cc}_test_checks.py

https://reviews.llvm.org/D69701 added support for on-the-fly argument
changes for update scripts. I recently wanted to keep some manual check
lines in a test generated by update_cc_test_checks.py in our CHERI fork, so
this commit adds support for UTC_ARGS in update_cc_test_checks.py. And since
I was refactoring the code to be in common.py, I also added it for
update_llc_test_checks.py.

Reviewed By: jdoerfert, MaskRay
Differential Revision: https://reviews.llvm.org/D78478