We somehow ended up with a set hidden in `Task` as well as a set at top level. SILGen currently
hooks into the top-level ones, so shed the `Task`-namespaced versions for now.
`get_async_continuation[_addr]` begins a suspend operation by accessing the continuation value that can resume
the task, which can then be used in a callback or event handler before executing `await_async_continuation` to
suspend the task.
The experimental concurrency model will require a supporting runtime
and possibly end-user-visible library constructs. Introduce a stub of
such a library, enabled by a new `build-script` option
`--enable-experimental-concurrency`, so we have a place to put this
work.
