4023: Fix another crash from wrong binders r=matklad a=flodiebold
Basically, if we had something like `dyn Trait<T>` (where `T` is a type parameter) in an impl we lowered that to `dyn Trait<^0.0>`, when it should be `dyn Trait<^1.0>` because the `dyn` introduces a new binder. With one type parameter, that's just wrong, with two, it'll lead to crashes.
Co-authored-by: Florian Diebold <flodiebold@gmail.com>
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