Handle CF <-> Objective-C toll-free-bridged conversions in the type checker.

Introduce an attribute that describes when a given CF type is
toll-free-bridged to an Objective-C class, and which class that
is. Use that information in the type checker to provide the CF <->
Objective-C toll-free-bridged conversions directly, rather than using
the user-defined conversion machinery.

Swift SVN r21376

lib/Serialization/Serialization.cpp

@@ -1315,6 +1315,7 @@ void Serializer::writeDeclAttribute(const DeclAttribute *DA) {
   case DAK_Ownership: // Serialized as part of the type.
   case DAK_Accessibility:
   case DAK_SetterAccessibility:
+  case DAK_ObjCBridged:
   case DAK_Count:
     llvm_unreachable("cannot serialize DAK_Count");
     return;