mark result string using String::mark_as_const()
which prevents CONCAT from reusing it as a buffer
for concatenation result.


mysql-test/r/information_schema.result:
  Bug#19599 duplication of information_schema column value in a CONCAT expr with user var
  test case
mysql-test/t/information_schema.test:
  Bug#19599 duplication of information_schema column value in a CONCAT expr with user var
  test case