Upgrade proc-macro2 to 1.0.51 am: 3cef6de22a

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/proc-macro2/+/2421165

Change-Id: Id87e116ba676fa71ef8a28316d2ee6fb589e0de4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>