Move call to store_tfeedback_info() into assign_varying_locations().

This will allow us to take advantage of the varying_match objects
(which only exist during execution of assign_varying_locations()) in
tfeedback_decl::store().
Remove tfeedback_decl::assign_location.  Instead, store the
association between tfeedback_decl and varying_match earlier in
linking, and then look up the location at the time of
tfeedback_decl::store().
Simplify the handling of gl_ClipDistanceMESA