[clang-tools-extra] [clang-tidy] Avoid processing declarations in system headers (PR #128150)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

> It should be doable, still defaulting to not skip system headers, and let clang-tidy skip them. A follow-up patch could change that default so all tools behave in the same way.


This does not sound controversial or hard. I am okay doing work that requires further consensus or a lot of investigation as a follow-up. But if that is not the case why not address it now? 

https://github.com/llvm/llvm-project/pull/128150