Debugging aid: Add T* List<T>::elem(int n) which returns N-th element in the list.

- There was List<N>::nth_element() but it didn't work because linker removed it. 
- Now, removal by linker is prevented for important values of T, and function is renamed.

sql/sql_test.cc

@@ -409,6 +409,15 @@ void print_sjm(SJ_MATERIALIZATION_INFO *sjm)
 }
 /* purecov: end */
 
+/*
+  Debugging help: force List<...>::elem function not be removed as unused.
+*/
+Item* (List<Item>:: *dbug_list_item_elem_ptr)(int)= &List<Item>::elem;
+Item_equal* (List<Item_equal>:: *dbug_list_item_equal_elem_ptr)(int)=
+  &List<Item_equal>::elem;
+TABLE_LIST* (List<TABLE_LIST>:: *dbug_list_table_list_elem_ptr)(int) =
+  &List<TABLE_LIST>::elem;
+
 #endif
 
 typedef struct st_debug_lock