[Runtime] Error bridged to NSError should return its description rather than NSError description

[theblixguy]

An Error bridged to NSError currently prints the NSError description, where as it should print the Error description. For example:

enum DecodingError: Error, CustomStringConvertible {
  case keyNotFound

  var description: String {
    switch self {
      case .keyNotFound: 
        return "Key not found"
    }
  }
}

// Prints: Error Domain=test.DecodingError Code=0 "(null)"
// instead of: "Key not found"
print((DecodingError.keyNotFound as NSError).description)

Resolves SR-7652

[mikeash]

Looks good, thanks!

[theblixguy]

@swift-ci please test

[theblixguy]

@swift-ci please test macOS