swift234

[Sema] Allow Optional and UncheckedOptional in @objc methods and properties.

...when the optional object type is a class or Objective-C protocol type.
This is currently hidden behind -enable-objc-optional because I didn't test
if the generated code is actually correct.

Also add PrintAsObjC support for these.

Known missing pieces: blocks, and Swift structs bridged to Objective-C classes
(right now, just String).

Swift SVN r14416