fix(backends/fstar): no __marker_trait if parent bounds

Parent bounds are translated as record fields in F*.  Also, F* classes
are records, and F* records cannot be empty. Thus, we insert a dummy
field `__marker_trait` on empty traits (marker traits).

On trait definitions, we were inserting this `__marker_trait` fields
only when a trait had no parent bounds and no associated item (method,
assoc. type, assoc fun.).

On instance definitons, we were inserting `__marker_trait` when the
trait was empty, disregarding parent bounds.

This difference of treatments lead to TC issues in F*.

This PR fixes this discrepancy.

engine/backends/fstar/fstar_backend.ml

@@ -1324,18 +1324,18 @@ struct
                        parent_bounds)
             items
         in
-        let fields =
-          if List.is_empty fields then
-            [ (F.lid [ "__marker_trait" ], pexpr (U.unit_expr e.span)) ]
-          else fields
-        in
         let parent_bounds_fields =
           List.map
             ~f:(fun (_impl_expr, impl_ident) ->
               (F.lid [ "_super_" ^ impl_ident.name ], F.tc_solve))
             parent_bounds
         in
         let fields = parent_bounds_fields @ fields in
+        let fields =
+          if List.is_empty fields then
+            [ (F.lid [ "__marker_trait" ], pexpr (U.unit_expr e.span)) ]
+          else fields
+        in
         let body = F.term @@ F.AST.Record (None, fields) in
         let tcinst = F.term @@ F.AST.Var FStar_Parser_Const.tcinstance_lid in
         F.decls ~fsti:ctx.interface_mode ~attrs:[ tcinst ]