swift234

Parse the new function syntax.

Parse the new function syntax, which allows both the argument (API)
and parameter (internal) name to be specified prior to the colon
within each parameter. Don't re-use the existing pattern-parsing
logic. Rather, implement a new (far simpler) parser for this purpose,
then map from its simple data structures to ArgParams and BodyParams
as we're used to.

There are a number of caveats here:
  - We no longer have the ability to use patterns for parameters in
  function declarations. The only place this really has an impact is
  that it makes the ~> hack in the standard library even uglier.
  - This exposed some issues with code completion with generic
  parameters that need to be investigated.
  - There's still some work to be done to improve recovery when things
  parse poorly within a parameter list; there are some XFAILs to deal
  with that.

I'll address the last two issues with follow-up commits.

Swift SVN r15967