MySQL manual describes values of the YEAR(2) field type as follows:
values 00 - 69 mean 2000 - 2069 years and values 70 - 99 mean 1970 - 1999
years. MIN/MAX and comparison functions was comparing them as int values
thus producing wrong result.

Now the Arg_comparator class is extended with compare_year function which
performs correct comparison of the YEAR type.
The Item_sum_hybrid class now uses Item_cache and Arg_comparator objects to
correctly calculate its value.
To allow Arg_comparator to use func_name() function for Item_func and Item_sum
objects the func_name declaration is moved to the Item_result_field class.
A helper function is_owner_equal_func is added to the Arg_comparator class.
It checks whether the Arg_comparator object owner is the <=> function or not.
A helper function setup is added to the Item_sum_hybrid class. It sets up
cache item and comparator.

mysql-test/r/func_group.result:
  Added a test case for the bug#43668.
mysql-test/t/func_group.test:
  Added a test case for the bug#43668.
sql/item.cc:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  Now Item_cache_int returns the type of cached item.
sql/item.h:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  To allow Arg_comparator to use func_name() function for Item_func and Item_sum
  objects the func_name declaration is moved to the Item_result_field class.
sql/item_cmpfunc.cc:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  The Arg_comparator class is extended with compare_year function which
  performs correct comparison of the YEAR type.
sql/item_cmpfunc.h:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  The year_as_datetime variable is added to the Arg_comparator class.
  It's set to TRUE when YEAR value should be converted to the
  YYYY-00-00 00:00:00 format for correct YEAR-DATETIME comparison.
sql/item_geofunc.cc:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  Item_func_spatial_rel::val_int chenged to use Arg_comparator's string
  buffers.
sql/item_subselect.h:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  Added an implementation of the virtual func_name function.
sql/item_sum.cc:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  The Item_sum_hybrid class now uses Item_cache and Arg_comparator objects to
  correctly calculate its value.
  A helper function setup is added to the Item_sum_hybrid class. It sets up
  cache item and comparator.
sql/item_sum.h:
  Bug#43668: Wrong comparison and MIN/MAX for YEAR(2)
  The Item_sum_hybrid class now uses Item_cache and Arg_comparator objects to
  correctly calculate its value.
  Added an implementation of the virtual func_name function.