Implement inferred_outlives_of for impl side RPITITs assoc type

Author: spastorino

compiler/rustc_ty_utils/src/assoc.rs

@@ -388,5 +388,8 @@ fn impl_associated_item_for_impl_trait_in_trait(
         predicates: &[],
     });
 
+    // There are no inferred outlives for the synthesized associated type.
+    impl_assoc_ty.inferred_outlives_of(&[]);
+
     local_def_id
 }