Method promotion_graph

cpp/src/arrow/json/reader.cc:128–128

  [[nodiscard]] MemoryPool* pool() const { return pool_; }
  [[nodiscard]] const ParseOptions& parse_options() const { return parse_options_; }
  [[nodiscard]] const PromotionGraph* promotion_graph() const { return promotion_graph_; }
  [[nodiscard]] const std::shared_ptr<DataType>& conversion_type() const {
    return conversion_type_;
  }