NS_SWIFT_BRIDGED_TYPEDEF - Why a typedef?

Just watched the "What's New in Cocoa" video, which mentioned NS_SWIFT_BRIDGED_TYPEDEF, which is now bridged to Swift as a String typedef instead of an explicit struct type, so that you can:

NSImage(named: "Foo")

Is there a reason that it was made a typedef, instead of just making the struct conform to ExpressibleByStringLiteral? I'm going to miss being to access system-provided images using the NSImage(named: .caution) syntax; NSImage(named: NSImage.cautionName) is not quite as concise.