libc++fs is part of libc++ now. am: e96ed9465f am: 82110fd0ac

Original change: https://android-review.googlesource.com/c/platform/external/puffin/+/3113981

Change-Id: Ie1beed17cdba75e53c2c23c173c381d26b1f0fe5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>