move value caching from get_datetime_value to fix_fields time

A followup for MDEV-9690.

Arg_comparator and other items were storing THD internally at fix_fields time and using it so that get_datetime_value could create an Item_cache_temporal at the first evaluation.

A correct fix for this bug is to move caching of temporal values from get_datetime_value to Arg_comparator::cache_converted_constant, where caches for non-temporal values are created at fix_fields time. This simplifies and unifies the code and further reduces the number of str_to_datetime calls.

work-in-progress patch is attached