According to dEQP this is an undefined section of the spec. Mark it as
unsupported for now until we can investgate if it's possible on
certain platforms.

Noticed this when testing with the Program refactor.

BUG=angleproject:1123
TEST=dEQP-GLES3.functional.transform_feedback.array_element.*

Change-Id: I830dddf418e28fb4536f09500f4069def1d97b58
Reviewed-on: https://chromium-review.googlesource.com/295984Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>