Sema: Disallow inheriting from classes with flexible array members

Flexible array members inherently index off of the end of their parent
type.

We shouldn't allow this type to be used as a base, virtual or otherwise,
because indexing off the end may find us inside of another base or the
derived types members.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193923 91177308-0d34-0410-b5e6-96231b3b80d8