Introduce the notion of factory initializers.

Factory initializers express an initializer that produces an object of
the given type, but is not inherited and not designated. Although they
have a syntactic form for presentation purposes (-> ClassName), there
is no way to specify or implement them within Swift. Rather, factory
initializers are created when importing an Objective-C factory method
that returns the class type rather than instancetype.

Swift SVN r16528

lib/Serialization/Serialization.cpp

@@ -1300,6 +1300,9 @@ getStableCtorInitializerKind(swift::CtorInitializerKind K){
   case swift::CtorInitializerKind::ConvenienceFactory:
     llvm_unreachable("Convenience factory initializers cannot be uttered");
 
+  case swift::CtorInitializerKind::Factory:
+    llvm_unreachable("Factory initializers cannot be uttered");
+
 #define CASE(NAME) \
   case swift::CtorInitializerKind::NAME: return serialization::NAME;
       CASE(Designated)