Added a test case for bug #12095.
sql_class.h:
  Fixed bug #12095: a join query with GROUP_CONCAT over a single row table.
  Added a flag to the TMP_TABLE_PARAM class forcing to put constant
  items generated after elimination of a single row table into temp table
  in some cases (e.g. when GROUP_CONCAT is calculated over a single row
  table).
  bk ci sql/item_sum.cc
  Fixed bug #12095: a join query with GROUP_CONCAT over a single row table.
  If GROUP_CONCAT is calculated we always put its argument into a temp
  table, even when the argument is a constant item.
sql_select.cc:
  Fixed bug #12095: a join query with GROUP_CONCAT over one row table.
  If temp table is used to calculate GROUP_CONCAT the argument should
  be always put into this table, even when it is a constant item.