[ms-cxxabi] Fix vbptr offsets in memptrs when the vbptr is in an nvbase

Also addresses a review comment from John from on r180985 by removing
the "== -1" check, since it's now reusing the correct code which has the
comment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183318 91177308-0d34-0410-b5e6-96231b3b80d8