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.
When build-script is given `--back-deploy-concurrency`, also use that
to build other parts of Swift with the back-deployed versions:
* The compiler allows async and actors to be defined with the
back-deployed availability, e.g., the same as `-Xfrontend
-enable-experimental-back-deploy-concurrency`. (The latter will go
away soon)
* The standard library unit testing framework and distributed actors
library are build with the older OS versions.
* The tests use the older OS versions, with some adjustments to make
them agnostic to the back-deployment setting.
We were not making references to async function pointers "weak" when
the function itself was weak, because we were always calculating
linkage as-if we were defining the async function pointer.
Fixes the rest of rdar://79674106.
