The biggest complexity here is dealing with the improper nesting of the
allocation for the argument, and I'm not very proud of the solution I found,
but for a one-off builtin, I think it'll work.
You can see in the patch that I obviously wrote this to include the
task-local builtins, but unfortunately they don't work for this because
they don't actually have a use/def relationship. That will need to be
follow-up work.
Includes a test that the defer special case applies to the priority
escalation builtin.
This interacts with my previous commits to ensure that we mark other
allocation operations as non-nested when we would otherwise need to move
the finishAsyncLet operation.
John McCall