Sema: Consider it an error to apply __builtin_offsetof to a member in a virtual base

icc 13 and g++ 4.9 both reject this while we would crash.

Fixes PR17578.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192674 91177308-0d34-0410-b5e6-96231b3b80d8