[Runtime] Avoid +class overrides when initializing an ObjC class.

swift_getInitializedObjCClass called [c class] to trigger class initialization, and returned the value. This wreaked havoc when the class in question overrides +class. Instead, ignore the return value and return c. Switch from +class to +self, which is much less likely to be overridden. Calling an overridden method could have performance downsides or even cause unwanted side effects.

rdar://problem/49853091

Author: mikeash

stdlib/public/runtime/SwiftObject.mm

@@ -1274,7 +1274,11 @@ id swift_dynamicCastObjCProtocolConditional(id object,
   // Used when we have class metadata and we want to ensure a class has been
   // initialized by the Objective-C runtime. We need to do this because the
   // class "c" might be valid metadata, but it hasn't been initialized yet.
-  return [c class];
+  // Send a message that's likely not to be overridden to minimize potential
+  // side effects. Ignore the return value in case it is overridden to
+  // return something different. See SR-10463 for an example.
+  [c self];
+  return c;
 }
 
 static const ClassMetadata *