gccrs: reuse an error mark node instead of new ones each failure

gccrs: reuse an error mark node instead of new ones each failure

During unification we denote failures using a new error mark node. This
keeps a static one and reuses it instead.

gcc/rust/ChangeLog:

	* typecheck/rust-unify.cc (unify_error_type_node): new static node
	(UnifyRules::go): likewise
	(UnifyRules::expect_inference_variable): likewise
	(UnifyRules::expect_adt): likewise
	(UnifyRules::expect_str): likewise
	(UnifyRules::expect_reference): likewise
	(UnifyRules::expect_pointer): likewise
	(UnifyRules::expect_param): likewise
	(UnifyRules::expect_array): likewise
	(UnifyRules::expect_slice): likewise
	(UnifyRules::expect_fndef): likewise
	(UnifyRules::expect_fnptr): likewise
	(UnifyRules::expect_tuple): likewise
	(UnifyRules::expect_bool): likewise
	(UnifyRules::expect_char): likewise
	(UnifyRules::expect_int): likewise
	(UnifyRules::expect_uint): likewise
	(UnifyRules::expect_float): likewise
	(UnifyRules::expect_isize): likewise
	(UnifyRules::expect_usize): likewise
	(UnifyRules::expect_never): likewise
	(UnifyRules::expect_placeholder): likewise
	(UnifyRules::expect_projection): likewise
	(UnifyRules::expect_dyn): likewise
	(UnifyRules::expect_closure): likewise
	(UnifyRules::expect_opaque): likewise
	(UnifyRules::expect_const): likewise

Signed-off-by: Philip Herron <herron.philip@googlemail.com>