This reverts commit b4b59726.

Reason for revert: This was in a chain of reverts, but is unrelated to the issue.

Original change's description:
> Revert "Vulkan: Move xfb position decl to translator in extension path"
>
> This reverts commit 8f5ca266.
>
> Reason for revert:
> Earlier CL breaks pre-rotation:
> https://chromium-review.googlesource.com/c/angle/angle/+/2598584
>
> Original change's description:
> > Vulkan: Move xfb position decl to translator in extension path
> >
> > This change removes the @@ XFB-DECL @@ marker.  The ANGLEXfbPosition
> > output is unconditionally emitted in VS, TES and GS by the translator,
> > and is appropriately decorated or removed by the SPIR-V transformer.
> >
> > Bug: angleproject:3606
> > Change-Id: Ia76224f5a6d147362eeb2d288f05e333aaf75481
> > Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2617658
> > Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
> > Reviewed-by: Jamie Madill <jmadill@chromium.org>
> > Reviewed-by: Charlie Lao <cclao@google.com>
>
> TBR=syoussefi@chromium.org,jmadill@chromium.org,cclao@google.com
>
> Change-Id: Ia03988b9c17639513576e82e8f11cd4c7b52640b
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: angleproject:3606
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2634202
> Reviewed-by: Tim Van Patten <timvp@google.com>
> Commit-Queue: Tim Van Patten <timvp@google.com>

TBR=timvp@google.com,syoussefi@chromium.org,jmadill@chromium.org,cclao@google.com

# Not skipping CQ checks because this is a reland.

Bug: angleproject:3606
Change-Id: Ib5b5925528a5c8698390b81f71ee788f5b332a1f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2633708
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>