Doug Gregor bb7a563e6c Switch async for-each loop over to _nextElement and drop @rethrows. ...

This couples together several changes to move entirely from
`@rethrows` over to typed throws:

* Use the `Failure` type to determine whether an async for-each loop
will throw, rather than depending on rethrows checking

* Introduce a special carve-out for `rethrows` functions that have a
generic requirement on an `AsyncSequence` or `AsyncIteratorProtocol`,
which uses that requirement's `Failure` type as potentially being part
of the thrown error type. This allows existing generic functions like
the following to continue to work:

    func f<S: AsyncSequence>(_: S) rethrows

* Switch SIL generation for the async for-each loop from the prior
`next()` over to the typed-throws version `_nextElement`.

* Remove `@rethrows` from `AsyncSequence` and `AsyncIteratorProtocol`
entirely. We are now fully dependent on typed throws.

2024-01-25 16:04:43 -08:00

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