This commit addresses column naming issues with CTEs in the use of prepared
statements and stored procedures. Usage of either prepared statements or
procedures with Common Table Expressions and column renaming may be affected.

There are three related but different issues addressed here.

1) First execution issue. Consider the following

prepare s from "with cte (col1, col2) as (select a as c1, b as c2 from t
order by c1) select col1, col2 from cte";
execute s;

After parsing, items in the select are named (c1,c2), order by (and group by)
resolution is performed, then item names are set to (col1, col2).
When the statement is executed, context analysis is again performed, but
resolution of elements in the order by statement will not be able to find c1,
because it was renamed to col1 and remains this way.

The solution is to save the names of these items during context resolution
before they have been renamed. We can then reset item names back to those after
parsing so first execution can resolve items referred to in order and group by
clauses.

2) Second Execution Issue

When the derived table contains more than one select 'unioned' together we could
reasonably think that dealing with only items in the first select (which
determines names in the resultant table) would be sufficient.  This can lead to
a different problem.  Consider

prepare st from "with cte (c1,c2) as
  (select a as col1, sum(b) as col2 from t1 where a > 0 group by col1
    union select a as col3, sum(b) as col4 from t2 where b > 2 group by col3)
  select * from cte where c1=1";

When the optimizer (only run during the first execution) pushes the outside
condition "c1=1" into every select in the derived table union, it renames the
items to make the condition valid.  In this example, this leaves the first item
in the second select named 'c1'.  The second execution will now fail 'group by'
resolution.

Again, the solution is to save the names during context analysis, resetting
before subsequent resolution, but making sure that we save/reset the item
names in all the selects in this union.

3) Memory Leak

During parsing Item::set_name() is used to allocate memory in the statement
arena.  We cannot use this call during statement execution as this represents
a memory leak.  We directly set the item list names to those in the column list
of this CTE (also allocated during parsing).

Approved by Igor Babaev <igor@mariadb.com>