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

[Alexander Richardson via llvm-commits]

arichardson wrote:

So it looks like this happens to work if we already mapped the function to another mangled name previously which is why the macho happens to work if we generated CHECK lines for ELF already.

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