swift234

Allow @objc protocols to be implemented with some (unsafe,
unchecked) laxity about optional types.

We really should be emitting thunks that do these checks; when
we emit those checks, we'll also be able to significantly broaden
the override/conformance rules to allow general subtyping, which
will give us a sensible language model here.  (Right now,
the "trivial override" rule would let us override a String
with a String? parameter --- but only in the bridged entrypoint...

Swift SVN r15778