swift234

Make ImplicitlyUnwrappedOptional conform to _ConditionallyBridgedToObjectiveC.

This is a better solution to <rdar://problem/16899681> because the
runtime magic is limited to implementing the witnesses of this
conformance.

The type checker fixes are because we can end up using unchecked
optionals in more places, via bridging, than we could before.


Swift SVN r18120