During statement preparation st_order::item gets set to a value in
ref_ptr_array. During statement execution we were overriding that value,
causing subsequent checks for window functions to return true.

Whenever we do any setting from ref_ptr_array, make sure to always
store the value in all_fields as well.

For function items containing window functions, as MDEV-12336 has
discovered, we don't need to create a separate Item_direct_ref or
Item_aggregate_ref as they will be computed directly from the top-level
item once the window function argument columns are computed.