Change Item::with_sum_func from a member to a virtual method

This task is similar to what we did earlier for Item::with_subselect (see MDEV-14517).

It's hard to track in the code which Item classes in the hierarchy:

• have always with_sum_func==true
• have always with_sum_func==false
• have variable with_sum_func

Under terms of this task we'll remove the member Item::with_sum_func and add a method instead:

virtual bool with_sum_func() const { return false; }

Item_sum will override this method as follows:

bool with_sum_func() const { return true; }

Only a few Item classes need a variable "with_sum_func" value:

• Item_ref
• Item_cache_wrapper
• Item_func
• Item_row
• Item_subselect

They'll store the "with_sum_func" value in a member, using a simple shared class With_sum_func_cache, which they will derive from.

Rationale:

• The code will be easier to read this way (instead of searching for all possible assignments of the member with_sum_func, one can just find who overrides the method)
• Also, Item types that do not need a variable "with_sum_func" will become potentially smaller in size (when alignment allows)