function.

A wrong  condition was used to check that the
Arg_comparator::can_compare_as_dates() function calculated the value of the
string constant. When comparing a non-const STRING function with a constant
DATETIME function it leads to saving an arbitrary value as a cached value of
the DATETIME function.

Now the Arg_comparator::set_cmp_func() function initializes the const_value
variable to the impossible DATETIME value (-1) and this const_value is
cached only if it was changed by the Arg_comparator::can_compare_as_dates()
function.


mysql-test/t/type_datetime.test:
  Added a test case for the bug#28208: Wrong result of a non-const STRING function with a const DATETIME function.
mysql-test/r/type_datetime.result:
  Added a test case for the bug#28208: Wrong result of a non-const STRING function with a const DATETIME function.
sql/item_cmpfunc.cc:
  Bug#28208: Wrong result of a non-const STRING function with a const DATETIME
  function.
  Now the Arg_comparator::set_cmp_func() function initializes the const_value
  variable to the impossible DATETIME value (-1) and this const_value is
  cached only if it was changed by the Arg_comparator::can_compare_as_dates()
  function.