glsl: Use the utility function to copy symbols between symbol tables

This effectively factorizes a couple of similar routines.

v2 (Neil Roberts): Non-trivial rebase on master

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>
Signed-off-by: Neil Roberts <nroberts@igalia.com>
(cherry picked from commit f5fe99ac)