Introduce `#if stdlib` or clarify `#if swift`

[tgoyne (Thomas Goyne)]

@Ben_Cohen

I'm not sure if it often works in unexpected ways. The introduction of primary associated types certainly caused confusion and it would be good to figure out a solution that avoids that confusion if something like this happens again in future. But as far as I remember, this is the one instance when this has been a noticeable problem (though maybe I'm forgetting something from previous years, apologies if so).

It's been a problem every year. The .0 releases of Xcode basically haven't been usable for macOS ever since the macOS release was pushed out of sync with the iOS release. I've never seen a reason to bother reporting bugs because it's not like it'd be fixed when it becomes irrelevant in .1 anyway.