Add PackedEnumBitSet, use it for buffer binding validation Includes angle::BitSetT changes from jmadill@chromium.org BUG=angleproject:2169 Change-Id: I9f896613f5c6cdc91281cb9a00134f67291870d9 Reviewed-on: https://chromium-review.googlesource.com/804177 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>