E2E snapshot test ObjC++ generator

Summary:
NativeModule specs exist under `react-native-github/packages/react-native-codegen/src/__tests__/modules/fixtures`. `GenerateModuleObjCpp-test.js` runs the RN Codegen on those NativeModule specs, and saves the output inside a snapshot. For convenience, the folowing command runs the legacy codegen on the fixtures:

```
buck build fbsource//xplat/js/react-native-github/packages/react-native-codegen/src/__tests__/modules:RNCodegenModuleFixtures-flow-types-ios --show-output
```

Changelog: [Internal]

Reviewed By: hramos

Differential Revision: D23637708

fbshipit-source-id: 3319f319515eca42b4499682313fea6e0bdc2a06