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

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

Change-Id: I96b03f76e96dc7a5d2207a184d7affd2826d608d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>