We expect to iterate on this quite a bit, both publicly
and internally, but this is a fine starting-point.
I've renamed runAsync to runAsyncAndBlock to underline
very clearly what it does and why it's not long for this
world. I've also had to give it a radically different
implementation in an effort to make it continue to work
given an actor implementation that is no longer just
running all work synchronously.
The major remaining bit of actor-scheduling work is to
make swift_task_enqueue actually do something sensible
based on the executor it's been given; currently it's
expecting a flag that IRGen simply doesn't know to set.
