If an attribute slot is unused and user enables it but doesn't bind any
buffer, Metal validation layer will complain that a buffer slot is
missing (even though the slot is not used inside shader).

Fix: if shader program doesn't use an attribute slot or there is no
buffer bound to it, then disable that attribute.

Bug: angleproject:2634
Change-Id: Ib56e0bf64bab13efce8b2972bfe386e26f45e50e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2392171
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>