Sema: Disallow derived classes with virtual bases from having flexible array members

Flexible array members only work out if they are the last field of a
record, however virtual bases would give us many situations where the
flexible array member would overlap with the virtual base fields.

It is unlikely in the extreme that this behavior was intended by the
user so raise a diagnostic instead of accepting.  This is will not
reject conforming code because flexible array members are an extension
in C++ mode.