Make the Program's semantic index D3D-only. This concept isn't strictly necessary for GL-side validation. Instead we can use a bitset to track active attribs, and determine is a particular location is active. BUG=angleproject:1123 Change-Id: If7a920a3071672116bafffb3368671f721723b65 Reviewed-on: https://chromium-review.googlesource.com/294570 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>