Issue Description:

I believe this is a Huge bug. In Xcode 8 we were given NS_STRING_ENUM and NS_EXTENSIBLE_STRING_ENUM. The former is suppose to turn objective-c Global strings into swift enum strings while the latter turns objective-c Global strings into swift structs with constant variable strings.

Steps to Reproduce:
Suppose we have the following objective-c header file
// traffic.h
typedef NSString * TrafficLightColor NS_STRING_ENUM;

static TrafficLightColor const TrafficLightColorRed = @"Red";
static TrafficLightColor const TrafficLightColorYellow = @"Yellow";
static TrafficLightColor const TrafficLightColorGreen = @"Green";

When we try to access TrafficLightColor in swift, it is treated as a struct. below is the Generated interface produced by Xcode:
struct TrafficLightColor : RawRepresentable, _SwiftNewtypeWrapper, Equatable, Hashable, Comparable, _ObjectiveCBridgeable {
static let red: TrafficLightColor
static let yellow: TrafficLightColor
static let green: TrafficLightColor
}

Expected Results:
According to the documentation, NS_STRING_ENUM should be ported to Swift as an enum.

Actual Results:
Although the documentation says NS_STRING_ENUM should be ported to swift as an enum. It actually gets ported over as a struct

Version:
Xcode 8.0 to 8.1

Notes:
Since NS_STRING_ENUM currently does not create an enum, it is not possible to create an exhaustive list in a switch statement. All switch statements evaluating it will need to end with a default case.

var trafficColor = TrafficLightColor.red
switch trafficColor {
case .red: ()
case .yellow: ()
case .green: ()
default🙁)
// Default case is required. If it was an enum, then all states would have been covered.
}

Notes:
I have 3 sources that claim an NS_STRING_ENUM should produce an enum:

1. Swift Evolution (SE-0033)
2. Xcode Quick Help
3. Apple documentation https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithCAPIs.html