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

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

Thank you for the feedback. I've made the requested changes.

Here's the new comment: https://github.com/nunoplopes/llvm2/pull/12#issuecomment-2517591665

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