swift234

QoI: Improve diagnostic for a missing required initializer with a Fix-It.

When a subclass is missing a required initializer, produce an error
within the subclass that mentions the required initializer along with
a Fix-It that provides an initializer stub, e.g.,

  required init(coder aDecoder: NSCoder!) {
      fatalError("init(coder:) has not been implemented")
  }

We take care to insert this stub in the main class, after all of the
initializers (if there are any) or near the beginning of the class (if
there aren't any initializers), and try to match the existing
indentation. If this works out, we should handle unsatisfied protocol
requirements the same way. <rdar://problem/17923210>

Swift SVN r21055