Update to syn-2 am: 4497fb4f59

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

Change-Id: Iabe6bafe2807371604177fafcf47e8066a83f0d8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>