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

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

I'd prefer to go ahead with this and continue the discussion once that PR landed. This would unbreak the checks/resolve the backward compatibility issues while providing some of the benefits (even if the perf gain is smaller). At least we will be in a state that does not break anything for anyone. And we can iterate from there. 

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