title: Enum initialized with a non-existent rawValue does not fail and return nil
body: <p>I have the following code in a playground (Xcode 9.0.1):</p>
  
  <pre><code>import MapKit
  
  enum Test: UInt {
      case first
      case second
      case third
  }
  
  let test = Test(rawValue: 4) as Any
  print(test)           // nil
  
  let type = MKMapType(rawValue: 999)
  print(type == nil)    // false
  print(type!.rawValue) // 999
  </code></pre>
  
  <p>MKMapType is defined as</p>
  
  <pre><code>enum MKMapType : UInt
  </code></pre>
  
  <p>As the maximum value of a MKMapType is 5, I expect the initializer of the enum to fail and return nil. Instead it returns 999. Am I missing some ObjC/Swift bridging here or could this be a bug?</p>