[ms] Fix vbtable index for covariant overrides of vbase methods

Overriding a method from a virtual base with a covariant return type
consumes a slot from the vftable in the virtual base. This can make it
impossible to implement certain diamond inheritance hierarchies, but we
have to follow along for compatibility in the simple cases.

This patch only affects our vtable dumper and member pointer function
mangling, since all other callers of getMethodVFTableLocation seem to
recompute VBTableIndex instead of using the one in the method location.

Patch by David Majnemer

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312017 91177308-0d34-0410-b5e6-96231b3b80d8