Making the Program own the attribs, and the Impl only see a read-only
copy cleans up the Impl object. It also allows us to more cleanly
isolate certain coded into D3D.

BUG=angleproject:1123

Change-Id: I469051eb066fc56e55282affa2d5398b394ab8d2
Reviewed-on: https://chromium-review.googlesource.com/293826Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>