c++: Template argument hashing [pr94454]

One of the problems hit by pr94454 was that the argument hasher was
not skipping nodes that template_args_equal would.  Fixed by replacing
the STRIP_NOPS invocation by a bespoke loop.  We also confuse the
canonical type machinery by treating 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.  That breaks the hasher because we'll use
TYPE_HASH (CANONICAL_TYPE ()) when we can. There's a note in
tsubst[TEMPLATE_TEMPLATE_PARM] about why the reduced ones cannot have
a canonical type. (I didn't feel like questioning that assertion at
this point.)

	* pt.c (iterative_hash_template_arg): Strip nodes as
	template_args_equal does.
	[ARGUMENT_PACK_SELECT, TREE_VEC, CONSTRUCTOR]: Refactor.
	[node_class:TEMPLATE_TEMPLATE_PARM]: Hash by level & index.
	[node_class:default]: Refactor.