Non-existent XFB varyings with the gl_ prefix used to pass linking on
the D3D11 backend. On a debug build they would cause an assert. Fix
these issues.

BUG=angleproject:2141
TEST=angle_end2end_tests

Change-Id: Iecc3d03823d02700d6b28c44d77df7a2f9e70a5b
Reviewed-on: https://chromium-review.googlesource.com/645747Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>