[MS ABI] Error instead of generating bad vftables for certain virtual hierarchies (PR18967)

Erroring out until we fix the bug means we don't have to keep chasing down
this same miscompile in a bunch of different places.

Differential Revision: http://llvm-reviews.chandlerc.com/D2890

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202331 91177308-0d34-0410-b5e6-96231b3b80d8