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.
When inferring the `Failure` associated type from the `next()` witness,
account for the fact that it might itself use typed throws. Use the
specified thrown error type for the inferred witness of `Failure`.
Fixes rdar://125446918.
