replace __nullable with _Nullable

Summary:
Modernize our null annotations
From the swift blog:
This feature was first released in Xcode 6.3 with the keywords nullable and nonnull. Due to potential conflicts with third-party libraries, we’ve changed them in Xcode 7 to the _Nullable and _Nonnull you see here.
drop-conflicts

Reviewed By: cuva

Differential Revision: D33121042

fbshipit-source-id: 821f5ec858d9afd5bfb1d6081c669f4ca18a36ed

React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.h

@@ -28,7 +28,7 @@ NS_ASSUME_NONNULL_BEGIN
 /*
  * Finds and returns the closet RCTScrollViewComponentView component to the given view
  */
-+ (RCTScrollViewComponentView *_Nullable)findScrollViewComponentViewForView:(UIView *)view;
++ (nullable RCTScrollViewComponentView *)findScrollViewComponentViewForView:(UIView *)view;
 
 /*
  * Returns an actual UIScrollView that this component uses under the hood.