MDEV-23330 Server crash or ASAN negative-size-param in my_strnncollsp_binary / SORT_FIELD_ATTR::compare_packed_varstrings

and
MDEV-23414 Assertion `res->charset() == item->collation.collation' failed in Type_handler_string_result::make_packed_sort_key_part

pack_sort_string() *must* take a collation from the Item, not from the
String value. Because when casting a string to _binary the original
String is not copied for performance reasons, it's reused but its
collation does not match Item's collation anymore.

Note, that String's collation cannot be simply changed to _binary,
because for an Item_string literal the original String must stay
unchanged for the duration of the query.

this partially reverts 61c15ebe