Accommodate [[nodiscard]] std::lock_guard in updated libc++ am: 164ecc7727 am: 25d1840a1e am: ea664191bd am: d7d78b1188 am: 36c4dfe35a

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

Change-Id: I48ad96b2e26a66f18bbad10618211816dece1aca
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>