glsl: fix recording of variables for XFB in TCS shaders

This is purely for conformance, since it's not actually possible to do
XFB on TCS output varyings. However we do have to make sure we record
the names correctly, and this removes an extra level of array-ness from
the names in question.

Fixes KHR-GL45.tessellation_shader.single.xfb_captures_data_from_correct_stage

v2: Add comment to the new program_resource_visitor::process function.
    (Ilia Mirkin)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108457


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: 19.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 4eec3a2a)