avoid ICE when extracting closure-kind-ty from a canonicalized value

In such a case, the `Infer` is converted to a `Bound`

Author: nikomatsakis

src/librustc/ty/sty.rs

@@ -2199,7 +2199,9 @@ impl<'tcx> TyS<'tcx> {
                 _ => bug!("cannot convert type `{:?}` to a closure kind", self),
             },
 
-            Infer(_) => None,
+            // "Bound" types appear in canonical queries when the
+            // closure type is not yet known
+            Bound(..) | Infer(_) => None,
 
             Error => Some(ty::ClosureKind::Fn),