swift234

Emit Objective-C thunks for [objc] initializing constructors.

Now that all [objc] constructors have a suitable "init" selector, emit
an Objective-C thunk for the initializing constructor (not the
allocating constructor!) with that selector, and make sure it shows up
in the Objective-C metadata. 

With this, we can write a Swift constructor "constructor()" to
override "-init"; see the change to ListMaker that exercises this.

Joe or John: I'd love a review of these.



Swift SVN r8373