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
Same deal as for opaque existentials--pre-instantiate a static witness table for one-witness-table types (the zero-witness-tables case is nicely handled by Builtin.ObjCPointer's value witness), and generate a vwtable using dynamic witness implementations for each different-sized container on demand as necessary.
Swift SVN r9850
We haven't fully updated references to union cases, and enums still are not
their own thing yet, but "oneof" is gone. Long live "union"!
Swift SVN r6783
