Fix a bug where using Message.Range alone does not clear
memory references for empty, but allocated lists and maps.
Thus, we iterate over every known field and explicitly call Clear.
Change-Id: I9c1847d4056cf66f3199947150f3140d0783444a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/183197
Reviewed-by: Damien Neil <dneil@google.com>
