Simplify exclusion of nested classes from extern template instantiation, NFC

Summary:
This simplifies three checks for MS ABI, Win Itanium, or Win GNU to just
"is Windows".

The question remains, however, if this is really the correct thing to
do. We could, for example, only not consider inner classes to be
externally available if the outer class has a dllexport annotation.
However, I will leave that as future work.

Reviewers: hans, mstorsjo

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61278

llvm-svn: 359507