swift

a8f3bb5e - Implement `AsyncIteratorProtocol.next()` in terms of `next(isolation:)`.

Implement `AsyncIteratorProtocol.next()` in terms of `next(isolation:)`. New async iterators should be able to implement only `next(isolation:)` and get the older `next()` implementation via a default. Implement the appropriate default witness. Fixes rdar://125447861.