Move codegen output out of node_modules

Summary:
In this diff, it moves the codegen output location out of node_modules and to build/generated/ios folder.

A temp pod spec will be created so that those files will be included in the Xcode project.

Changelog: [Internal]

Reviewed By: hramos, cortinico

Differential Revision: D31809012

fbshipit-source-id: ba1c884c8024306ba0fd2102837b7dbebc6e18ac

packages/rn-tester/NativeModuleExample/ScreenshotManager.podspec

@@ -35,6 +35,5 @@ Pod::Spec.new do |s|
     :react_native_path => "../../..",
     :js_srcs_dir => "./",
     :library_type => "modules",
-    :output_dir => "./"
   })
 end