Pass GPU events instead of streams across async regions.

Lower !gpu.async.tokens returned from async.execute regions to events instead of streams.

Make !gpu.async.token returned from !async.execute single-use.
This allows creating one event per use and destroying them without leaking or ref-counting.
Technically we only need this for stream/event-based lowering. I kept the code separate
from the rest of the gpu-async-region pass so that we can make this optional or move
to a separate pass as needed.

Reviewed By: herhut

Differential Revision: https://reviews.llvm.org/D96965