[Runtime] Add ObjC support to isKnownUniquelyReferenced.

Add code to support detecting uniquely referenced Objective-C and Core
Foundation objects.

rdar://47902425
rdar://66805490

lib/IRGen/IRGenModule.cpp

@@ -802,6 +802,17 @@ namespace RuntimeConstants {
     }
     return RuntimeAvailability::AlwaysAvailable;
   }
+
+  RuntimeAvailability
+  ObjCIsUniquelyReferencedAvailability(ASTContext &context) {
+    auto featureAvailability =
+        context.getObjCIsUniquelyReferencedAvailability();
+    if (!isDeploymentAvailabilityContainedIn(context, featureAvailability)) {
+      return RuntimeAvailability::ConditionallyAvailable;
+    }
+    return RuntimeAvailability::AlwaysAvailable;
+  }
+
 } // namespace RuntimeConstants
 
 // We don't use enough attributes to justify generalizing the