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

[Hans Wennborg]

  I realized this actually doesn't cover my reduction in http://llvm.org/bugs/show_bug.cgi?id=18967#c6

  Maybe we can just drop the if-statement that checks whether the base is primary or not?

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