gccrs: simplify cloning of types during unify

gccrs: simplify cloning of types during unify

We should only clone when we commit and it was successful. With no
cloning there is a regression in min specialization 2. Probably because
it needs to know the unify site hirid and ensure we have it set there.

gcc/rust/ChangeLog:

	* typecheck/rust-unify.cc (UnifyRules::Resolve): check for success
	(UnifyRules::expect_inference_variable): dont clone
	(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

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