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

[Nikolas Klauser via llvm-commits]

philnik777 wrote:

This flags `#undef`: https://github.com/llvm/llvm-project/pull/118167#issuecomment-2557175936.

@nunoplopes please fix this soon, since it's quite confusing for new contributors.

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