Revert "Revert r260388 "[MS ABI] Never reference dllimport'd vtables""

This reverts commit r260449.

We would supress our emission of vftable definitions if we thought
another translation unit would provide the definition because we saw an
explicit instantiation declaration.  This is not the case with
dllimport, we want to synthesize a definition of the vftable regardless.

This fixes PR26569.

llvm-svn: 260548