[C++20] [Modules] Handle linkage properly for specializations when overloading

Currently, the semantics of linkage in clang is slightly
different from the semantics in C++ spec. In C++ spec, only names
have linkage. So that all entities of the same should share
one linkage. But in clang, different entities of the same could
have different linkage.

It would break a use case where the template have external linkage and
its specialization have internal linkage due to its type argument is
internal linkage. The root cause is that the semantics of internal
linkage in clang is a mixed form of internal linkage and TU-local in
C++ spec. It is hard to solve the root problem and I tried to add a
workaround inplace.