Summary:
Use the ObjC++ native modules files generated by `react-native-codegen` for the FBReactNativeSpec library.
People working with the core native modules may now re-generate these ObjC++ files using `USE_CODEGEN=1 pod install` or `scripts/generate-native-modules-specs.sh`.
Changelog: [Internal]
Reviewed By: RSNara
Differential Revision: D22249978
fbshipit-source-id: a12db23defacdba36099a24ce0b8475c8973bd6d