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

[Piotr Zegar via cfe-commits]

https://github.com/PiotrZSL approved this pull request.

I like what I see.
One thing that may be required later is manual review of implementation of every check to verify that they will work correctly, current tests may not be sufficient.

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