Having any problems in Swift getting access to native definitions through Command-click? You’re not alone. (As one person put it, “b6 is [colloquial expression of extreme disappointment] even by Xcode beta standards”) Here’s a workaround.
Select any Foundation item like NSString and Command-click it like you used to. In the definitions file, use the Jump bar to move from Foundation > NSString > NSString to Swift.
Update: If Command-Click is not working within source code, try quitting/restarting Xcode (thanks Argyrios K.). Still borked for me in Playgrounds.
This brings you to the Swift definitions you were trying to reach. Copy them to a file somewhere for easy reference.