[llvm] Add PR check to suggest alternatives to using undef (PR #118506)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> nunoplopes Just using the word "undef" in a C++ comment will also trigger this. I think is very confusing.

Have you tested this? It should be specific to `.ll` files.

https://github.com/llvm/llvm-project/pull/118506