Support emitting typed RCTDeviceEmitter events

Summary:
This enables to code-gen base C++ types for custom exported JS types from a RN TM spec - which have been previously excluded from code-gen as these aren't used in any function.

The only work around so far was to ‘register’ a random function using the custom type which should be used for RCTDeviceEventEmitter events

Changelog: [Internal]

Reviewed By: rshest

Differential Revision: D56685903

fbshipit-source-id: add9ca40018b91c9fca98609ba3d1f85d3affec1