Generate RN specific Files in RN (#47458)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47458

We are in a weird situation where React Native depends on some files that are generated by Codegen.

Codegen runs in the user project, so those dependencies are not available to React Native if we try to build it in isolation.

This is a problem and a blocker to prepare the prebuilds for iOS.

This image show the changes we are introducing:
On the right we have the current situation.
On the left the new one.

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## Changelog:
[Internal] - Generate React Native specific code inside React Native

Reviewed By: cortinico, blakef

Differential Revision: D65541505

fbshipit-source-id: 1412d7f23c4d2230b795af41f1e832c8a70d5859