[MS ABI] Detect and diagnose vftables which cannot be created

The MSVC ABI has a bug introduced by appending to the end of vftables
which come from virtual bases: covariant thunks introduces via
non-overlapping regions of the inheritance lattice both append to the
same slot in the vftable.

It is possible to generate correct vftables in cases where one node in
the lattice completely dominates the other on the way to the base with
the vfptr; in all other cases, we must raise a diagnostic in order to
prevent the illusion that we succeeded in laying out the vftable.

This fixes PR16759.

llvm-svn: 236354