[MS ABI] Initialize "most general" member pointers which don't point at a vbase

The most general model has fields for the vbptr offset and the vbindex.
Don't initialize the vbptr offset if the vbindex is 0: we aren't
referencing an entity from a vbase.

Getting this wrong can make member pointer equality fail.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240043 91177308-0d34-0410-b5e6-96231b3b80d8