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

[Benjamin Maxwell via llvm-commits]

MacDue 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
```


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