Accommodate [[nodiscard]] std::lock_guard in updated libc++ am: 164ecc7727

Original change: https://android-review.googlesource.com/c/platform/system/incremental_delivery/+/2674657

Change-Id: Ic2d2c46e294c70b6a9e37d7054c649c1062a3a3d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>