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

[Baranov Victor via cfe-commits]

vbvictor wrote:

If modules are considered as system headers in clang-tidy, there was work in https://github.com/llvm/llvm-project/pull/128150 to reduce scope of traversal to match only in user code (I suppose it would affect modules too). But that PR had to be reverted due to appeared issues with downstream users. Hopefully it will reland in the future. @carlosgalvezp may shine some light on this matter.

Treating modules separately doesn't seem right to me.
Also, If we exclude only `decls` what happens with `stmts` and other nodes, are they still getting matched and filtered? What happens if a check looks for a `decl` inside system header and later compare it to `decl` inside user-code?

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