adding test case
item_strfunc.cc:
  Bug#12351
  CONCAT with USER()/DATEBASE() and
  a column gets strange results.
  Mark created Item_str as constant, so CONCAT
  cannot reuse it for optimization purposes.


sql/item_strfunc.cc:
  Bug#12351
  CONCAT with USER()/DATEBASE() and
  a column gets strange results
mysql-test/t/func_system.test:
  adding test case
mysql-test/r/func_system.result:
  adding test case