Problem: GROUP_CONCAT(DISTINCT BIT_FIELD...) uses a tree to store keys;
which are constructed using a temporary table fields,
see Item_func_group_concat::setup().
As a) we don't store null bits in the tree where the bit fields store parts 
of their data and b) there's no method to properly compare two table records
we've got problem.

Fix: convert BIT fields to INT in the temporary table used.


mysql-test/r/func_gconcat.result:
  Fix for bug #31154: field.h:1649: virtual int Field_bit::cmp(const uchar*, const uchar*): Assertion
    - test result.
mysql-test/t/func_gconcat.test:
  Fix for bug #31154: field.h:1649: virtual int Field_bit::cmp(const uchar*, const uchar*): Assertion
    - test case.
sql/item_sum.cc:
  Fix for bug #31154: field.h:1649: virtual int Field_bit::cmp(const uchar*, const uchar*): Assertion
    - force the create_tmp_table() to convert BIT columns to INT 
      in order to be able to compare records containing BIT fields.