[MS ABI] Correctly handle dllimport'd explicit instantiation declaration w/ vbases

We gave a VBTable dllimport storage class and external linkage while
also providing an initializer.  An initializer is only valid if the
VBTable has available_externally linkage.  Fix this by setting the
linkage to available_externally in situ while generating the
initializer.

This fixes PR26686.

llvm-svn: 261535