Use memoizedState in componentDidUpdate

We forgot to clone this value so it didn't work before.
This is covered by existing tests in ReactDOMProduction.

src/renderers/shared/fiber/ReactFiber.js

@@ -253,6 +253,7 @@ exports.cloneFiber = function(fiber : Fiber, priorityLevel : PriorityLevel) : Fi
   alt.pendingWorkPriority = priorityLevel;
 
   alt.memoizedProps = fiber.memoizedProps;
+  alt.memoizedState = fiber.memoizedState;
   alt.output = fiber.output;
 
   return alt;

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -289,9 +289,7 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) {
         } else {
           if (typeof instance.componentDidUpdate === 'function') {
             const prevProps = current.memoizedProps;
-            // TODO: This is the new state. We don't currently have the previous
-            // state anymore.
-            const prevState = instance.state || null;
+            const prevState = current.memoizedState;
             instance.componentDidUpdate(prevProps, prevState);
           }
         }