swift234

Two related but (theoretically at least) seperable changes:
1. Implement parser and sema support for our subscript syntax proposal in
   protocols.  Now you have to use subscript(..) { get } or  {get set} to 
   indicate what you want.  I suspect that the syntax will evolve, but at
   least we can express what we need now.
2. Change the representation of SubscriptDecls in protocols to make 
   (empty) funcdecls for the getter and setter.  This guarantees that 
   every subscript has at least a getter.



Swift SVN r12555