[C++2x][Modules] Amend module purview constant linkage [P2788R0].

This paper has been applied to the working draft and is believed to be
a DR against C++20, so that the patch here makes the change unconditionally.

for:
```
export module A;

const int mod_cst = 10;
```
Before the change, mod_cst would have internal linkage; after the change it
has module linkage.

Differential Revision: https://reviews.llvm.org/D145886