swift234

[SE-0112] Import an Objective-C error enum as a struct wrapping NSError.

A given Objective-C error enum, which is effectively an NS_ENUM that
specifies its corresponding error domain, will now be mapped to an
ErrorProtocol-conforming struct that wraps an NSError, much like
NSCocoaError does. The actual enum is mapped to a nested "Code"
enum. For example, CoreLocation's CLError becomes:

  struct CLError : ErrorProtocol {
    let _nsError: NSError
    // ...
   @objc enum Code : Int {
     case ...
   }
  }

This implements bullet (2) in the proposed solution of SE-0112, so
that Cocoa error types are mapped into structures that maintain the
underlying NSError to allow more information to be extracted from it.