PR c++/35722
	Implement N2555 (expanding pack expansion to fixed parm list)
	* pt.c (coerce_template_parms): Allow expanding a pack expansion
	to a fixed-length argument list.
	(unify_pack_expansion): Handle explicit args properly.
	(unify) [TREE_VEC]: Handle pack expansions here.
	[TYPE_ARGUMENT_PACK]: Not here.
	(tsubst_pack_expansion): Don't try to do partial substitution.
	(pack_deducible_p): New.
	(fn_type_unification): Use it.
	(find_parameter_packs_r): Take the TYPE_MAIN_VARIANT
	of a type parameter.
	(check_non_deducible_conversion): Split from type_unification_real.
	(unify_one_argument): Split from type_unification_real...
	(unify_pack_expansion): ...and here.  Drop call_args_p parm.
	(type_unification_real, unify, more_specialized_fn): Adjust.

From-SVN: r179436