Fix decltype-call1.C with -std=c++1z.

Fix decltype-call1.C with -std=c++1z.

	* parser.h (struct cp_token): Tell GTY that CPP_DECLTYPE uses
	tree_check_value.
	* parser.c (cp_parser_decltype): Use tree_check_value.
	(saved_checks_value): New.
	(cp_parser_nested_name_specifier_opt): Use it.
	(cp_parser_template_id): Use it.
	(cp_parser_simple_type_specifier): Use it.
	(cp_parser_pre_parsed_nested_name_specifier): Use it.

From-SVN: r231353