MDEV-7245: Incorrect display of subquery cache in union.result

- Do set nest_level and nest_level_base for fake_select_lex objects.

mysql-test/r/union.result

@@ -1617,7 +1617,7 @@ NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	NULL	Using filesort
 3	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 Warnings:
 Note	1276	Field or reference 'a' of SELECT #3 was resolved in SELECT #-1
-Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` union select `test`.`t1`.`a` AS `a` from `test`.`t1` order by <expr_cache><>((select `a` from `test`.`t2` where (`test`.`t2`.`b` = 12)))
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` union select `test`.`t1`.`a` AS `a` from `test`.`t1` order by <expr_cache><`a`>((select `a` from `test`.`t2` where (`test`.`t2`.`b` = 12)))
 # Should not crash
 SELECT * FROM t1 UNION SELECT * FROM t1
 ORDER BY (SELECT a FROM t2 WHERE b = 12);

sql/sql_parse.cc

@@ -7757,6 +7757,9 @@ bool st_select_lex_unit::add_fake_select_lex(THD *thd_arg)
   fake_select_lex->context.resolve_in_select_list= TRUE;
   fake_select_lex->context.select_lex= fake_select_lex;  
 
+  fake_select_lex->nest_level_base= first_select()->nest_level_base;
+  fake_select_lex->nest_level=first_select()->nest_level;
+
   if (!is_union())
   {
     /*