swift234

Move CallExpr diagnostics over to the same overload candidate diagnosis 
facilities used by operators etc.  This required a bunch of changes to make
the diagnostics changes strictly an improvement:

  - Teach the new path about calls to TypeExprs.
  - Teach evaluateCloseness some simple things about varargs.
  - Make the generic diagnosis logic produce a better error when there is 
    exactly one match.

Overall, the resultant diagnostics are a step forward: we now produce candidate
set notes more uniformly, and the messages about some existing ones are 
more specific.  This is just another stepping stone towards progress though.



Swift SVN r30057