Fix parsing of parenthesised OLD/NEW in RETURNING list.

Fix parsing of parenthesised OLD/NEW in RETURNING list.

When parsing expressions like (old).colname and (old).* in a RETURNING
list, the parser would lose track of the intended varreturningtype,
and therefore return incorrect results.

The root cause was code using GetNSItemByRangeTablePosn() to find a
namespace item from its rtindex and levelsup, without taking into
account returningtype, which would return the wrong namespace item.
Fix by adding a new function GetNSItemByVar() that does take
returningtype into account.

Backpatch to v18, where support for RETURNING OLD/NEW was added.

Bug: #19516
Reported-by: Marko Grujic <markoog@gmail.com>
Author: Marko Grujic <markoog@gmail.com>
Suggested-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: https://postgr.es/m/CAOvwyF2cO_5mAt=w=y-dFnaG5UkZ+3H8nSDoKF_iuWZHsU2ARg@mail.gmail.com
Backpatch-through: 18