IN NESTED CONDITION

Analysis
========
Aggregate function in certain nested queries does have
context set to aggregate select instead of bsae select
of the item and leads to unwanted behaviour during merge
context.

The problem lies within Item::split_sum_func2(), where an
Item_aggregate_ref object that wraps the set function is created.
It is created with context.select_lex equal to the aggregation
query block, and depended_from is the same query block,
when this query block is different from the "base" query block
of the set function.

Fix
====
Created the Item_aggregate_ref with context.select_lex set
to be the "base" query block of the set function, and depended_from
as the aggregation query block.

[This is a backport of a change done in 8.0 to fix Bug#17270896:
Crash in Item_ident::fix_after_pullout()]