[MS ABI] Reuse getVFPtrOffsets instead of using getClassAtVTableLocation

getClassAtVTableLocation hunts through virtual bases without using the
MDC layout which is indicative of a bug.

Instead, reuse the getVFPtrOffsets machinery to calculate which
subobject within the MDC is responsible for the vfptr.

Differential Revision: https://reviews.llvm.org/D25895

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285315 91177308-0d34-0410-b5e6-96231b3b80d8