[flow] Make NominalT for CustomError behave more like OpenT during union optimization

Summary:
This diff makes `NominalT` for custom error purposes behave more like OpenT. Notably, we will mark it as needing further concretization and can be flattened, so that it can participate in union optimizations.

Changelog: [internal]

Reviewed By: gkz

Differential Revision: D86676840

fbshipit-source-id: 13b0a3818006ea87816d0b22a4876e6abae52117

Author: SamChou19815

src/typing/flow_js_utils.ml

@@ -246,7 +246,8 @@ let needs_resolution = function
   | UnionT _
   | OptionalT _
   | MaybeT _
-  | AnnotT _ ->
+  | AnnotT _
+  | NominalT (_, { underlying_t = Nominal.CustomError _; _ }) ->
     true
   | _ -> false
 

src/typing/type_mapper.ml

@@ -32,6 +32,7 @@ let union_flatten =
       let void_t = VoidT.why_with_use_desc ~use_desc r in
       void_t :: flatten cx seen t
     | DefT (_, EmptyT) -> []
+    | NominalT (_, { underlying_t = Nominal.CustomError { t; _ }; _ }) -> flatten cx seen t
     | EvalT { type_ = _; defer_use_t = TypeDestructorT (_, _, ValuesType); id } ->
       (match Eval.Map.find_opt id (Context.evaluated cx) with
       | Some cached_t -> flatten cx seen cached_t