A method of CASE searched/simple detection

MariaDB changed to using separate classes for simple/searched in 10.3. ColumnStore needs to figure out which is being used. We would consider something like using dynamic_cast on the item but the server has no-rtti so this fails.

Instead can we please have a virtual function in Item_func_case similar to this?

virtual const char* case_type()= 0;

This would return something like "simple" for simple classes and "searched" for searched classes.