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/+/2634202Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>