swift234

Update 'oneof' syntax.

Give oneof bodies syntax consistent with other NominalTypes. Give oneof elements first-class declaration syntax using the 'case' introducer, as suggested by Jordan. Oneofs can contain 'case' decls, functions, properties, and constructors, but not physical ivars. Non-oneof scopes cannot contain 'case' decls. Add some QoI to the oneof 'case' parser to also parse and complain about things that resemble switch 'case' labels inside decl contexts.

Swift SVN r6211