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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

Hmm, it does not remove much

- Baseline: `Suppressed 196093 warnings (196093 in non-user code)`
- Trunk: `Suppressed 8050 warnings (8050 in non-user code).` 
  - Apparently there's a bug in the `isSystemHeader` function, since some core compiler headers like `os_defines.h` are *not* classified as system, and `clang-tidy` still processes those).
- Proposal: `Suppressed 141779 warnings (141779 in non-user code).`


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