swift234

Fix crash during type checker diagnostic emission.

We were crashing attempting to diagnose the case where we cannot assign
an array literal to a type conforming to _ArrayProtocol because we don't
know the element types of the conforming type.

We still don't give a great diagnostic for this, so I've
opened (rdar://problem/27594154) to track improving it.

Resolves rdar://problem/25563498.