message: Add missing unreachable to exhaustive switch statement
diff: @@ -70,6 +70,8 @@ std::string BSONCXX_CALL to_json(document::view view, ExtendedJsonMode mode) {
  case ExtendedJsonMode::k_canonical:
  return to_json_helper(view, bson_as_canonical_extended_json);
  }
  +
  + BSONCXX_UNREACHABLE;
  }
  document::value BSONCXX_CALL from_json(stdx::string_view json) {
  