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

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

> It's good practice to keep commits small, focused on doing one single thing


True. But doing this in another component will not make the patch larger. So this is not an argument against. Moreover, the scope is not bigger than this patch. It would not change the behavior of other tools by default. Only introduce an option that other tools can adopt on their own accord.

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