[MS ABI] Form member pointers from virtual funcs overriding vbases

We didn't supporting taking the address of virtual member functions
which overrode a method in a virtual base.  We simply need to encode the
virtual base index in the member pointer.

This fixes PR23452.

N.B.  There is no data member pointer side to this change because taking
the address of a virtual bases' data member gives you a member pointer
whose type is derived from the virtual bases' type, not the most derived
type.

llvm-svn: 236962