Add a type cast to RCT_ENUM_CONVERTER for C++ compatibility
Add a type cast to RCT_ENUM_CONVERTER for C++ compatibility

Summary:
C++ doesn't provide an implicit cast to an enum value from the enum's backing type. When a `.mm` file calls `RCT_ENUM_CONVERTER`, we end up with the following compiler error:
> Error: cannot initialize return object of type `<TypeName>` with an rvalue of type `NSInteger`

Since `RCT_ENUM_CONVERTER` is a macro, this error will appear whenever we try to expand the macro in a C++ context.

The project compiles and runs as expected when this additional cast is added 😃
Closes https://github.com/facebook/react-native/pull/14408

Reviewed By: javache

Differential Revision: D5215584

Pulled By: jballer

fbshipit-source-id: 7f4464afd7cd9dc9a014f646278bae20731f08ba

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