PR c++/80396 - built-in for make_integer_sequence.

	* pt.c (builtin_pack_fn_p, builtin_pack_call_p)
	(expand_integer_pack, expand_builtin_pack_call): New.
	(find_parameter_packs_r): Check builtin_pack_call_p.
	(check_for_bare_parameter_packs): Handle it.
	(tsubst_pack_expansion): Call expand_builtin_pack_call.
	(declare_integer_pack): New.
	(init_template_processing): Call it.
	* decl2.c (mark_used): Check builtin_pack_fn_p.

From-SVN: r248384