[C++20] [Modules] Treat module linkage as formal linkage only

Close https://github.com/llvm/llvm-project/issues/61321

There are two linkage modes in clang now: one for internal linkage and
one for formal linkage. The internal linkage is for the implementation
details and the formal linkage is for the consistency with the C++
standard.

Since we previously implemented the strong ownership for modules, the
module linkage is not meaningful for linkers any more. So the module
linkage should only be used for formal linkage.