* [cxx-interop] Add Hashable conformance to imported enums
Previously, imported enums only conformed to RawRepresentable and Equatable,
so they could not be used as members of a Set or keys of a Dictionary.
This patch adds Hashable conformance to give them that ability,
as well as some test cases to clarify the expected behavior.
Existing test cases are updated to reflect this new conformance.
rdar://129713687
