[ms-cxxabi] Fix assert in unspecified inheritance memptr emission

If a class is using the unspecified inheritance model for member
pointers and later we find the class is defined to use single
inheritance, zero out the vbptr offset field of the member pointer when
it is formed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192664 91177308-0d34-0410-b5e6-96231b3b80d8