Sema: Diagnose undefined structs used as Microsoft anonymous structs

Previously, we would not mark structs containing anonymous structs as
invalid.  Later, horrific things would occur when trying to determine
the size of the parent record.

Instead, require the struct to be a complete type when used as an
anonymous struct.  Mark both the anonymous field for the struct and the
parent context as invalid (this is similar to what we do when a struct
contains a field with an incomplete type.)

This fixes PR11847.