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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

I've rebased on latest main now and intend to land it as soon as the CI checks pass. Let me know if I should do any last-minute changes before that! Thank you all for the review :) 

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