[UpdateTestChecks] Fix `update_*_test_checks.py` to add "unused" prefixes

The support introduced in D124306 was only added to
update_llc_test_checks.py, but the motivating usecases (see
https://lists.llvm.org/pipermail/llvm-dev/2021-February/148326.html)
cover update_test_checks.py, update_cc_test_checks.py, and
update_analyze_test_checks.py, too.

Issue #59220.

Differential Revision: https://reviews.llvm.org/D138836