MDEV-27277 Add a warning when max_sort_length is reached

During a query execution some sorting and grouping operations
on strings may be involved. System variable max_sort_length defines
the maximum number of bytes to use when comparing strings during
sorting/grouping. Thus, the comparable parts of strings may be less
than their actual size, so the results of the query may be not
sorted/grouped properly.
To indicate that some comparisons were done on a truncated lengths,
a new warning has been introduced with this commit.

sql/sql_class.cc

@@ -1386,6 +1386,7 @@ void THD::init()
   first_successful_insert_id_in_cur_stmt= 0;
   current_backup_stage= BACKUP_FINISHED;
   backup_commit_lock= 0;
+  num_of_strings_sorted_on_truncated_length= 0;
 #ifdef WITH_WSREP
   wsrep_last_query_id= 0;
   wsrep_xid.null();