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
We're going to remove the support for modules-ts. But there are a lot of
tests which uses -fmodules-ts. We shouldn't remove them simply. This
patch refactor these tests to use standard c++ modules.