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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

I think this patch can be orthogonal to the system headers one, the original one from @njames93 contained similar logic I believe (but I chose to not include it for simplicity).

It's probably good to apply this change now before modules become mainstream and we have debt to deal with, like it happened with my patch :) 

I agree however that there may be other things than Decls to take care of, but I don't have a complete picture here. 

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