c++: Remove SET_PACK_EXPANSION_PATTERN / SET_ARGUMENT_PACK_ARGS

c++: Remove SET_PACK_EXPANSION_PATTERN / SET_ARGUMENT_PACK_ARGS

Unlike in C, in C++ the conditional operator yields an lvalue if both
branches are lvalues, so these setter macros are unnecessary; we can
just assign to PACK_EXPANSION_PATTERN and ARGUMENT_PACK_ARGS directly.

gcc/cp/ChangeLog:

	* coroutines.cc (instantiate_coro_traits): Adjust accordingly.
	* cp-tree.def: Remove mention of SET_PACK_EXPANSION_PATTERN.
	* cp-tree.h (SET_PACK_EXPANSION_PATTERN): Remove.
	(SET_ARGUMENT_PACK_ARGS): Remove.
	* module.cc (trees_in::tree_node): Adjust accordingly.
	* parser.cc (make_char_string_pack): Likewise.
	(make_string_pack): Likewise.
	* pt.cc (make_pack_expansion): Likewise.
	(template_parm_to_arg): Likewise.
	(coerce_template_parameter_pack): Likewise.
	(extract_fnparm_pack): Likewise.
	(extract_locals_r): Likewise.
	(make_argument_pack): Likewise.
	(tsubst_argument_pack): Likewise.
	(lookup_init_capture_pack): Likewise.
	(type_unification_real): Likewise.
	(unify_pack_expansion): Likewise.
	(tsubst_initializer_list): Likewise.