swift234

IRGen/Runtime: Expose extra inhabitants of class types.

Start using null-page values as extra inhabitants when laying out single-payload enums that contain class pointers as their payload type. Don't use inhabitants that set the lowest bit, to avoid trampling potential ObjC tagged pointer representations. This means that 'T?' for class type T now has a null pointer representation. Enums with multiple empty cases, as well as nested enums like 'T??', should now have optimal representations for class type T as well.

Note that we don't yet expose extra inhabitants for aggregates that contain heap object references, such as structs with class fields, Swift function types, or class-bounded existentials (even when the existential has no witness tables).

Swift SVN r10061