[clang] [clang-tools-extra] [clang-tidy] Avoid matching nodes in system headers (PR #151035)

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

I'm happy to split the little refactor of `shouldSkipNode` into a separate patch to make this one smaller / not have to revert the whole thing if it doesn't work out, please let me know :) 

https://github.com/llvm/llvm-project/pull/151035