cache_left_rec on rule ending in underscore

[QnnOkabayashi]

When you have a rule with #[cache_left_rec] and the rule name ends in an underscore (e.g. type_), you get a warning saying "Field type__cache should have a snake_case name". Since this is an implementation detail, I imagine that the macro should add #[allow(non_snake_case)] on the generated helper rule.