The active attributes are those consumed by the Program. Previously
we were returning those enabled as client arrays. But this excluded
default attributes.

Uncovered when changing how resource usage was tracked for Vulkan
multithreading.

Bug: angleproject:2464
Change-Id: I48996be9e4470bb1432e042f98046c95ea8adbfe
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1808718
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>