Group variants only if obtainable via \cs_generate_variant:Nn

Previously, \clist_count:n and \clist_count:N were treated as
variants.  Now hopefully not anymore.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5334 de43f980-851b-0410-b2f7-c40aca1f87e0