[Concurrency] Actors & actor isolation

[beccadax (Becca Royal-Gordon)]

@Douglas_Gregor

@Lantua

Wouldn't we need a separated axis for that? In particular, how would it work with DispatchQueue.concurrentPerform which is concurrent (and so racing) and non-escaping

We might need to introduce yet another kind of function type for "concurrent, non-escaping". There's more design to do here, and we need to be mindful of the amount of complexity it might introduce.

I'm pretty sure we do need another value here for "actor-confined"; withoutActuallyEscaping(_:do:)'s documentation explicitly discusses using it to run non-escaping closures concurrently, and changing our minds about that being legal seems tantamount to a source break.