Also fixes:
MDEV-11331 Wrong result for INSERT INTO t1 (datetime_field) VALUES (hybrid_function_of_TIME_data_type)
MDEV-11333 Expect "Impossible where condition" for WHERE timestamp_field>=DATE_ADD(TIMESTAMP'9999-01-01 00:00:00',INTERVAL 1000 YEAR)

This patch does the following:
1. Splits the function Item::save_in_field() into pieces:
- Item::save_str_in_field()
- Item::save_real_in_field()
- Item::save_decimal_in_field()
- Item::save_int_in_field()

2. Adds the missing "no_conversion" parameters to
   Item::save_time_in_field() and Item::save_date_in_field(),
   so this parameter is now correctly passed to
   set_field_to_null_with_conversions().
   This fixes the problem reported in 11333.

3. Introduces a new virtual method Type_handler::Item_save_in_field()
   and uses the methods Item::save_xxx_in_field() from the implementations
   of Type_handler_xxx::Item_save_in_field().

These changes additionally fix the problem reported in MDEV-11331,
as the old code erroneously handled expressions like
COALESE(datetime-expression) through the STRING_RESULT branch of
Item::save_in_field() and therefore they looked like string type expressions
for the target fields. Now such expressions are correctly handled by
Item::save_date_in_field().