Update to syn-2 am: 4497fb4f59 am: a2ad5a30b1 am: 63b69f1f39

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/async-stream-impl/+/2522575

Change-Id: Idd991c8a0e23fc126242c542278cc29844c02cbb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>