[C++20] [Modules] Exit early if export decl is not valid

This patch fixes a crash due to following simple program:
> export struct Unit {
>    bool operator<(const Unit&);
> };

It would crash since the compiler would set the module ownership for
Unit. And the declaration with a module ownership is assumed to own a
module. But here isn't one. So here is the crash.

This patch fixes this by exiting early if it finds the export decl is
already invalid.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D117093