c++: Harden *_PACK_EXPANSION and *_ARGUMENT_PACK macros

c++: Harden *_PACK_EXPANSION and *_ARGUMENT_PACK macros

This makes the accessor macros for TYPE_/EXPR_PACK_EXPANSION
and TYPE_/NONTYPE_ARGUMENT_PACK check the tree code of the argument.

gcc/cp/ChangeLog:

	* cp-tree.h (PACK_EXPANSION_CHECK): Define.
	(PACK_EXPANSION_PATTERN): Use PACK_EXPANSION_CHECK.
	(SET_PACK_EXPANSION_PATTERN): Likewise.
	(PACK_EXPANSION_PARAMETER_PACKS): Likewise.
	(PACK_EXPANSION_EXTRA_ARGS): Likewise.
	(PACK_EXPANSION_LOCAL_P): Likewise.
	(PACK_EXPANSION_SIZEOF_P): Likewise.
	(PACK_EXPANSION_AUTO_P): Likewise.
	(PACK_EXPANSION_FORCE_EXTRA_ARGS_P): Likewise.
	(ARGUMENT_PACK_CHECK): Define.
	(ARGUMENT_PACK_ARGS): Use ARGUMENT_PACK_CHECK.
	(SET_ARGUMENT_PACK_ARGS): Likewise.
	* parser.cc (cp_parser_sizeof_pack): Check for error_mark_node
	before setting PACK_EXPANSION_SIZEOF_P.