[modules] If a referenced-but-not-instantiated class template specialization

[modules] If a referenced-but-not-instantiated class template specialization
gets explicitly specialized, don't reuse the previous class template
specialization declaration as a new declaration. The benefit here is fairly
marginal, it harms source fidelity, and this is horrible to model if the
specialization was imported from another module (without this change, it
asserts or worse).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209552 91177308-0d34-0410-b5e6-96231b3b80d8