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

[via cfe-commits]

tonygould wrote:

This is huge. @carlosgalvezp thank you so much for all the hard work on the development and getting this merged. I love clang-tidy, but the build time has made it very difficult to use (and socialize) on large projects, especially for CICD.

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