Sema: Do not assert when dereferencing member pointer using virtual inheritance with an incomplete class type

The MS ABI requires that we determine the vbptr offset if have a
virtual inheritance model.  Instead, raise an error pointing to the
diagnostic when this happens.

This fixes PR18583.

Differential Revision: http://llvm-reviews.chandlerc.com/D2842


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201824 91177308-0d34-0410-b5e6-96231b3b80d8