Revisiting SE-0282: distributing swift-atomics with the toolchain

[Joe_Groff]

@lorentey

After three long years, non-copyable types are now becoming a real thing. However, modeling synchronization primitives isn't a flagship use case for them; in fact, properly modeling these things continues to be an elusive thing.

I don't think this is true. Once the implementation of SE-0390 is mature, then we're really only missing one thing necessary to model atomics or locks with move-only types, namely the ability to put a block of raw memory inside such a type. Figuring that out, either in the general case or in a special case just for atomics, would be a great flagship use for move-only types.