gccrs: Add check bounds flag to unify rules for compatability checks

gccrs: Add check bounds flag to unify rules for compatability checks

We need to make the type bounds check a flag because it can turn into a
recursive type bounds check. This allows us to remove another can_eq usage

gcc/rust/ChangeLog:

	* typecheck/rust-type-util.cc (types_compatable):  add check bounds flag
	(unify_site_and): likewise
	* typecheck/rust-type-util.h (types_compatable): likewise
	(unify_site_and): likewise
	* typecheck/rust-tyty-bounds.cc: likewise
	* typecheck/rust-unify.cc (UnifyRules::UnifyRules): likewise
	(UnifyRules::Resolve): likewise
	(UnifyRules::resolve_subtype): likewise
	(UnifyRules::go): likewise
	* typecheck/rust-unify.h: likewise

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