Return a FxIndexSet in is_late_bound query.

This return value is iterated upon by borrowck, hence the need to preserve
a deterministic iteration order.

compiler/rustc_middle/src/query/mod.rs

@@ -1573,7 +1573,7 @@ rustc_queries! {
         Option<&'tcx FxHashMap<ItemLocalId, Region>> {
         desc { "looking up a named region" }
     }
-    query is_late_bound_map(_: LocalDefId) -> Option<&'tcx FxHashSet<LocalDefId>> {
+    query is_late_bound_map(_: LocalDefId) -> Option<&'tcx FxIndexSet<LocalDefId>> {
         desc { "testing if a region is late bound" }
     }
     /// For a given item (like a struct), gets the default lifetimes to be used