- MDEV-8875 Wrong metadata for MAX(CAST(time_column AS DATETIME))
- MDEV-8873 Wrong field type or metadata for LEAST(int_column,string_column)
- MDEV-8912 Wrong metadata or type for @c:=string_or_blob_field

Adding Item_hybrid_func as a common parent for Item_func_hybrid_field_type,
Item_func_min_max, Item_func_user_var. This removes some duplicate code.