Get RN Podspec xcodeproject to compile

Summary:
This diff includes a few changes to the `React.podspec` file:
1. Introduce a `jsi` spec for code inside the `ReactCommon/jsi` folder. This depends on the JavaScriptCore framework.
2. Introduce a `jsiexecutor` spec for the code inside the `ReactCommon/jsiexecutor` folder. These files depend on files in `ReactCommon/cxxreact`, `ReactCommon/jsi`, and Folly.
3. Since RCTCxxBridge.mm now depends on `JSIExecutor`, we need to have the `CxxBridge` spec depend on the `jsiexecutor` spec.

Reviewed By: hramos

Differential Revision: D9820323

fbshipit-source-id: 0c96d027eed30ee47b6ee0d2d86cd6b1ad7a5887