[libc++] Fix extern template visibility for Windows

On Windows, marking an `extern template class` declaration as exported
actually forces an instantiation, which is not the desired behavior.
Instead, the actual explicit instantiations need to be exported.

Differential Revision: https://reviews.llvm.org/D24679

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@281925 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed