Fix shift_bound_vars

It should only shift free vars (maybe the name isn't the best...)

crates/ra_hir_ty/src/lib.rs

@@ -814,13 +814,13 @@ pub trait TypeWalk {
     where
         Self: Sized,
     {
-        self.fold(&mut |ty| match ty {
+        self.fold_binders(&mut |ty, binders| match ty {
-            Ty::Bound(idx) => {
+            Ty::Bound(idx) if idx as usize >= binders => {
                 assert!(idx as i32 >= -n);
                 Ty::Bound((idx as i32 + n) as u32)
             }
             ty => ty,
-        })
+        }, 0)
     }
 }