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

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

> This flags `#undef`: [#118167 (comment)](https://github.com/llvm/llvm-project/pull/118167#issuecomment-2557175936).
> 
> @nunoplopes please fix this soon, since it's quite confusing for new contributors.

Regex tweaked here: https://github.com/llvm/llvm-project/commit/0c629206b0b3e1ddd64104a56c7a573a55e8944a
Please let me know if come across other cases that need tweaking. Thanks!

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