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

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

> I have not seen any failing tests for these checks.

Those checks might be downstream. LLVM is usually OK breaking downstream users but we should still make some effort helping them out documenting the rationale and suggesting workarounds. 

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