swift234

rework SubscriptDecls to work the same as VarDecls: first the 
SubscriptDecl is created, then the accessors are installed on it.
This allows us to create the subscript decl before the accessors
have been parsed, allowing us to build the subscript even in invalid
cases (better for later error recovery).

More importantly, this allows us to add it to Decls before calling
parseGetSet, so we can now make parseGetSet add accessors to Decls
without breaking source order (something that deeply upsets the IDE
features).

With all this untangled, we can now remove the 'addAccessorsInOrder'
hack where we parsed the accessors and then later tried to figure out
which order they came for the purpose of linking up the AST: accessors
now work just like everything else.



Swift SVN r13708