If an enum has no cases with payloads, make it implicitly Equatable and Hashable, and derive default implementations of '==' and 'hashValue'. Insert the derived '==' into module context wrapped in a new DerivedFileUnit kind, and arrange for it to be codegenned with the deriving EnumDecl by adding a 'DerivedOperatorDecls' array to NominalTypeDecls that gets visited at SILGen time.
Swift SVN r14471
