swift234

The Payload case of SinglePayloadEnumImplStrategy is more complicated than
I thought.  Apparently there is no way to directly test whether a general
enum contains the payload without testing against each of the potential 
inhabitant cases with a switch.  Instead of solving this open problem, just
optimize the case where we're testing for a single non-payload case, and 
special case enums with one payload and one non-payload case (hello optional!)
by testing for the non-payload case and inverting the result.  This resolves
the miscompilations I was seeing before with my full patch set, but is still
NFC since the code that uses this isn't included yet.




Swift SVN r24365