swift234

Make argument names default to keyword arguments in the cases where Objective-C has names.

Introduce a model where an argument name is a keyword argument if: 

  - It is an argument to an initializer, or
  - It is an argument to a method after the first argument, or
  - It is preceded by a back-tick (`), or
  - Both a keyword argument name and an internal parameter name are
    specified. 

Provide diagnostics Fix-Its to clean up cases where the user is
probably confused, i.e.,

  - "_ x: Int" -> "x: Int" where "x" would not have been a keyword
  argument anyway
  - "x x: Int" -> "`x: Int"

This covers the compiler side of <rdar://problem/16741975> and
<rdar://problem/16742001>.

Update the AST printer to print in this form, never printing just 
a type for a parameter name because we're also going to adopt
<rdar://problem/16737312> and it was easier to move the tests once
rather than twice.

Standard library and test updates coming separately.




Swift SVN r17056