The only piece of code that's needed to be generated for the extension
path is the following, at the right spot (right before depth correction
and pre-rotation):

    ANGLEXfbPosition = gl_Position;

The SPIR-V transformer already has gl_Position loaded for depth
correction and pre-rotation, so this change simply adds an OpStore to
ANGLEXfbPosition.

As a result of this change, @@ XFB-OUT @@ is no longer emitted if
the transform feedback extension is supported.  With the above code now
placed correctly for geometry shaders, transform feedback tests for
geometry shaders are enabled.

Bug: angleproject:3606
Change-Id: I13a7956ab62a1a6b4196ff999442b99b50226c0f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2617659
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>