swift234

[Clang Importer] implement swift_newtype(enum) as struct

This is a temporary solution that implements swift_newtype(enum) as
though it were written swift_newtype(struct). This is to work around
to the fact that a String-backed enum does not actually have a String
stored, and a struct is closer to reflecting that storage
properly. Struct provides most of the functionality and appearance for
now, though it does not allow for switching over the values.

Full support for swift_newtype(enum) as a Swift enum is forthcoming.