with null values

For queries containing GROUP_CONCAT(DISTINCT fields ORDER BY fields), there 
was a limitation that the DISTINCT fields had to be the same as ORDER BY 
fields, owing to the fact that one single sorted tree was used for keeping 
track of tuples, ordering and uniqueness. Fixed by introducing a second 
structure to handle uniqueness so that the original structure has only to 
order the result.


mysql-test/r/func_gconcat.result:
  Bug#32798:
  - Wrong test result turned correct after fix.
  - Correct test result
mysql-test/t/func_gconcat.test:
  Bug#32798: Test case
sql/item_sum.cc:
  Bug#32798: Implementation of fix. Dead code removal.
  
  - removed comment describing this bug
  - replaced body of function group_concat_key_cmp_with_distinct
  - removed function group_concat_key_cmp_with_distinct_and_order
  - Added a Unique object to maintain uniqueness of values.
sql/item_sum.h:
  Bug#32798: Declarations and comments.