swift234

Tricky selector-style parameter parsing into creating ParamDecls.

The selector-style parameter parsing code is going away "soon", but we
still need to prop it up a bit longer. Hence, I don't feel too bad
about the Parser-level state I'm using in this hack to make it happen.

With that change, we can now establish two important invariants in the
AST:
  - Only parameters (ParamDecl or GenericTypeParamDecl) can have their
  DeclContexts changed. Everything else comes into being in the
  correct context.
  - All of the parameters in a function/constructor/closure/etc. are
  described by ParamDecls, not just VarDecls.



Swift SVN r16593