Use String instead of NSString
Use String instead of NSString

Summary: `String` and `NSString` bridging has been improved in recent Swift versions. Serialization of a `JSON` enum to a string should use the `String` struct instead of `NSString`.

Reviewed By: marekcirkos

Differential Revision: D4550966

fbshipit-source-id: cc902033263ecb27f51d5a27849ea9343826e413

StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,plu/FBSimulatorControl