Updates to SELECT_LEX::merge_subquery

A check in Item_ident::collect_outer_ref_processor ensures that we do not traverse into a different unit when collecting outer references. This nest_level_base pointer may need updating when we merge as subquery SELECT_LEX with a different nest_level_base into a SELECT_LEX.

Once we have merged subq_select into this SELECT_LEX, we need to walk into the subquery SELECT_LEX, correcting nest_level_base.

We also update Item_singlerow_subselect::select_transformer() for the same reason.

Item_subselect::get_cache_parameters calls Item::collect_outer_ref_processor. This processor only includes items that have the same nest_level_base as the enclosing unit. For this example

create table t1 (a int not null, b int not null);

create table t2 (c int not null, d int not null);

create table t3 (e int not null);

select a from t1 where a in (select c from t2 where d >= some(select e from t3 where b=e));

t1.b, buried in the lowest level select has a different nest_base_level to the lowest level Item_subselect after transforms.

select t3.e from t3 where t1.b = t3.e and (t2.d) >= t3.e

This results in an incorrect answer due to a missing subquery cache parameter.