When temporary tables is used for result sorting
result field for gconcat function is created using
group_concat_max_len size. It leads to result truncation
when character_set_results is multi-byte character set due
to insufficient tmp table field size.
The fix is to increase temporary table field size for
gconcat. Method make_string_field() is overloaded
for Item_func_group_concat class and uses
max_characters * collation.collation->mbmaxlen size for
result field. max_characters is maximum number of characters
what can fit into max_length size.


mysql-test/r/ctype_utf16.result:
  test result
mysql-test/r/ctype_utf32.result:
  test result
mysql-test/r/ctype_utf8.result:
  test result
mysql-test/t/ctype_utf16.test:
  test case
mysql-test/t/ctype_utf32.test:
  test case
mysql-test/t/ctype_utf8.test:
  test case
sql/item.h:
  make Item::make_string_field() virtual
sql/item_sum.cc:
  added Item_func_group_concat::make_string_field(TABLE *table) method
  which uses max_characters * collation.collation->mbmaxlen size for
  result item. max_characters is maximum number of characters what can
  fit into max_length size.
sql/item_sum.h:
  added Item_func_group_concat::make_string_field(TABLE *table) method