[C++20] [Module] fix bug 47716 and implement [module.interface]/p6

This fixes bug 47716.

According to [module.interface]p2, it is meaningless to export an entity
which is not in namespace scope.
The reason why the compiler crashes is that the compiler missed
ExportDecl when the compiler traverse the subclass of DeclContext. So
here is the crash.

Also, the patch implements [module.interface]p6 in
Sema::CheckRedeclaration* functions.

Reviewed By: aaron.ballman, urnathan

Differential Revision: https://reviews.llvm.org/D112903