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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

>  I think hasAncestor/hasParent and co should still be able to reach nodes from system headers.

I think the naive expectation is that these would be able to reach nodes in system headers, so I agree that it would be good to see if we can improve the behavior here. CC @carlosgalvezp @PiotrZSL @HerrCai0907 

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