When a C++ foreign reference type is conformed to a Swift protocol via a Swift extension, trying to cast `any MyProtocol` to the foreign reference type crashes the runtime.
This was because `selectCasterForDest` wasn't handling C++ foreign reference types, and we were hitting `swift_unreachable`.
This change makes sure the runtime doesn't crash for such casts.
Notably, Swift doesn't have enough metadata to determine if the conditional cast actually succeeded. This is also a problem for CF types. Casting CF types in a similar fashion triggers a typechecker diagnostic. That diagnostic will be amended in a follow-up patch to also trigger for foreign reference types.
rdar://141227849
