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

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

Ok, I added a link to langref. The message itself gives an example on how to avoid the most common undef mistake.

I think the patch is a good starting point to go live. We can work through more documentation afterwards, I think.

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