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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/79c65b9d97fe92ea2792be09479cf4bbea7cefe1

Modified Files
--------------
src/backend/parser/parse_coerce.c | 9 ++++-----
src/backend/parser/parse_func.c | 4 +---
src/backend/parser/parse_relation.c | 32 ++++++++++++++++++++++++++++++++
src/backend/parser/parse_target.c | 2 +-
src/include/parser/parse_relation.h | 1 +
src/test/regress/expected/returning.out | 25 +++++++++++++++++++++++++
src/test/regress/sql/returning.sql | 11 +++++++++++
7 files changed, 75 insertions(+), 9 deletions(-)