Method has_explicit_value

sql/field.h:1295–1298

    bitmap_set_bit(&table->has_value_set, field_index);
  }
  bool has_explicit_value() const
  {
    return bitmap_is_set(&table->has_value_set, field_index);
  }
  void clear_has_explicit_value()
  {
    bitmap_clear_bit(&table->has_value_set, field_index);