c++: tpl-tpl-parms are not canonicalizable types [pr94454]

c++: tpl-tpl-parms are not canonicalizable types [pr94454]

We treat tpl-tpl-parms as types.  They're not; bound-tpl-tpl-parms
are.  We can get away with them being type-like.  Unfortunately we
give the original level==orig_level case a canonical type, but the
reduced cases of level<orig_level get structural equality.  This patch
gives them structural type always.

	* pt.c (canonical_type_parameter): Assert not a tpl-tpl-parm.
	(process_template_parm): tpl-tpl-parms are structural.
	(rewrite_template_parm): Propagate structuralness.