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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> It would be good to have a unit test of this problem, would the one in the CERT check mentioned above be sufficient?

I think it's pretty reasonable.

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