Fix another crash from wrong binders

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.

crates/ra_hir_ty/src/lib.rs

@@ -396,12 +396,12 @@ impl Substs {
     }
 
     /// Return Substs that replace each parameter by a bound variable.
-    pub(crate) fn bound_vars(generic_params: &Generics) -> Substs {
+    pub(crate) fn bound_vars(generic_params: &Generics, debruijn: DebruijnIndex) -> Substs {
         Substs(
             generic_params
                 .iter()
                 .enumerate()
-                .map(|(idx, _)| Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, idx)))
+                .map(|(idx, _)| Ty::Bound(BoundVar::new(debruijn, idx)))
                 .collect(),
         )
     }