Packager linking when using custom debug configuration on iOS device (#16451)

Summary:
In many projects we have different environments that we connect to. For these environments (for example: Test, Staging, Production) we have custom configurations (Debug and Release). While this is not a problem on Android, it is a problem on iOS. With the current implementation of the react-native-xcode.sh script, when using a custom Debug configuration, the app started on iOS device, can't contact the Packager. This pull request solves this issue.

Connect a real device, start the app with a custom debug configuration in Xcode. Shake and Reload.

[IOS][BUGFIX][./scripts/react-native-xcode.sh] Add support for connecting to the Packager when running the iOS app on device when using custom Debug configuration.
Closes https://github.com/facebook/react-native/pull/16451

Differential Revision: D8730537

Pulled By: hramos

fbshipit-source-id: a36007776e8fe9e401c38015040abd2c2bbd7c58