[Interop][SwiftToCxx] Emit helper cases class in C++ for Swift enum

[WANGJIEKE]

Emit Swift enum cases inside generated C++ class. For example, following Swift enum

public enum Foo { case x, y, z }

will have cases in generated C++ header file like this

class Foo final {
public:
  enum class cases { x, y, z };
};

@hyp Could you review this pull request?

[hyp]

@swift-ci please test

[hyp]

@swift-ci please test source compatibility

[hyp]

Looks good. The only thing I was wondering about if the cases should be printed out separately on new lines, instead if all being on the same line. Wdyt?

[WANGJIEKE]

Yeah maybe it's better to print one line per case so that if there's an enum with lots of cases, it can be more human-readable. I think it's typically easier to scroll vertically than horizontally.

[hyp]

@swift-ci please test

[hyp]

@swift-ci please test source compatibility