- The bug was caused by COUNT(DISTINCT ...) code using Unique object in 
  a way that assumed that BIT(N) column occupies a contiguous space in
  temp_table->record[0] buffer. 
- The fix is to make COUNT(DISTINCT ...) code instruct create_tmp_table to
  create temporary table with column of type BIGINT, not BIT(N).


mysql-test/r/type_bit.result:
  BUG#30324: Grouping queries with COUNT(DISTINCT bit column) return wrong results
  - Testcase
mysql-test/t/type_bit.test:
  BUG#30324: Grouping queries with COUNT(DISTINCT bit column) return wrong results
  - Testcase
sql/item_sum.cc:
  BUG#30324: Grouping queries with COUNT(DISTINCT bit column) return wrong results
  - Make COUNT(DISTINCT ...) code instruct create_tmp_table to create 
    temporary table with BIGINT, not BIT(N) column.