Bug#28133: Wrong DATE/DATETIME comparison in IN() function.

The IN function was comparing DATE/DATETIME values either as ints or as
strings. Both methods have their disadvantages and may lead to a wrong
result.

Now IN function checks whether all of its arguments has the STRING result
types and at least one of them is a DATE/DATETIME item. If so it uses either
an object of the in_datetime class or an object of the cmp_item_datetime
class to perform its work. If the IN() function arguments are rows then
row columns are checked whether the DATE/DATETIME comparator should be used
to compare them.
The in_datetime class is used to find occurence of the item to be checked
in the vector of the constant DATE/DATETIME values. The cmp_item_datetime
class is used to compare items one by one in the DATE/DATETIME context.
Both classes obtain values from items with help of the get_datetime_value()
function and cache the left item if it is a constant one.

mysql-test/r/func_in.result

@@ -467,6 +467,7 @@ CREATE TABLE t4 (a DATE);
 INSERT INTO t4 VALUES ('1972-02-06'), ('1972-07-29');
 SELECT * FROM t4 WHERE a IN ('1972-02-06','19772-07-29');
 a
+1972-02-06
 Warnings:
 Warning	1292	Incorrect date value: '19772-07-29' for column 'a' at row 1
 DROP TABLE t1,t2,t3,t4;