[clang] [llvm] Handle leading underscores in update_cc_test_checks.py (PR #121800)

[Alexander Richardson via cfe-commits]

https://github.com/arichardson requested changes to this pull request.

A few suggestions but this should be good to land once formatting has been fixed and the baseline test is in.

I'd also drop the regen of the neon intrinsics from this patch and do that separately.

https://github.com/llvm/llvm-project/pull/121800