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

[Alexander Richardson via llvm-commits]

arichardson wrote:

I've been looking into this a bit and I can see that this works for for most function but for some reason fails with NEON vector type arguments/return values. Even though the mangled name is identitical. I will pre-commit some tests for this.

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