[C++20] [Modules] Namespace Declaration shouldn't have module linkage

According to [basic.namespace.general]/p2, a namespace declaration
shouldn't have a module linkage.
> A namespace is never attached to a named module and never has a name
> with module linkage.

Without this patch, the compiler would crash for the test in assertion
enabled build due to inconsistent linkage for redeclaration for
namespaces.

Reviewed by: rsmith

Differential Revision: https://reviews.llvm.org/D115132