These new Clang attributes identify whether an enum is intended to
represent an option set or not, and whether the set of cases listed in
the enum declaration is exhaustive. (Swift doesn't currently have a
closed/open distinction for enums, so treat any C enum with
enum_extensibility as a proper closed Swift enum, like we do with
NS_ENUM.)
Enums with neither attribute will continue to be imported as unique
types.
rdar://problem/28476618
Enum constants are naturally going to be named after their ObjC name,
not their Swift name. As such, ignore the swift_name attr on the enum
decl when calculating the common prefix. It turns out this is actually
simpler anyway as it also bypasses the swift_private handling that the
code was already trying to work around.
