swift234

Clean up the import of Objective-C init methods as Swift initializers.

Previously, we would import the init method as a FuncDecl, which
wasn't actually supposed to be used by anything, and then created a
separate ConstructorDecl via importSpecialMember(). That's insane:
just have VisitObjCMethodDecl() produce a ConstructorDecl directly
when given an init method. Should be NFC.

Swift SVN r16313