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

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

> @nunoplopes Just using the word "undef" in a C++ comment will also trigger this, e.g.:
> 
> ```
> // Using undef in a comment will trigger the bot
> ```

it is impossible to come up with a regex that will cover all cases. Nevertheless, writing undef is probably wrong. Maybe people want to write undefined behavior (UB) or poison. undef should be avoided as it's misleading.
All warnings generated by tools are subject to false positives; it's up to the reviewers to exercise their best judgement.

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