[AST] [Modules] Introduce Decl::getNonTransparentDeclContext to handle exported friends

Closing https://github.com/llvm/llvm-project/issues/56826.

The root cause for pr56826 is: when we collect the template args for the
friend, we need to judge if the friend lives in file context. However,
if the friend lives in ExportDecl lexically, the judgement here is
invalid.

The solution is easy. We should judge the non transparent context and
the ExportDecl is transparent context. So the solution should be good.

A main concern may be the patch doesn't handle all the places of the
same defect. I think it might not be bad since the patch itself should
be innocent.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D131651