[clang] [clang-tools-extra] [clang-tidy] [Modules] Skip checking decls in clang-tidy (PR #145630)

[Baranov Victor via cfe-commits]

https://github.com/vbvictor commented:

> I think modules are not system headers. For the example in the issue, the module interface is not system nor headers. It is another TU but we just can get AST from it.

Indeed, they are handled different.
Left a minor comment.

https://github.com/llvm/llvm-project/pull/145630