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

[Alexander Richardson via llvm-commits]

arichardson wrote:

Could you give me an example of such an architecture? Isn't it a bug that clang reports an incorrect mangled name?
Also please add a test for this (ideally pre-committing one that fails before and passes with this change).

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