pgsql: Track nesting depth correctly when drilling down into RECORD Var

Track nesting depth correctly when drilling down into RECORD Vars.

expandRecordVariable() failed to adjust the parse nesting structure
correctly when recursing to inspect an outer-level Var. This could
result in assertion failures or core dumps in corner cases.

Likewise, get_name_for_var_field() failed to adjust the deparse
namespace stack correctly when recursing to inspect an outer-level
Var. In this case the likely result was a "bogus varno" error
while deparsing a view.

Per bug #18077 from Jingzhou Fu. Back-patch to all supported
branches.

Richard Guo, with some adjustments by me

Discussion: /messages/by-id/18077-b9db97c6e0ab45d8@postgresql.org

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/2679a107a152de3f88602a41f8651ec82e4d6205

Modified Files
--------------
src/backend/parser/parse_target.c | 20 ++++++++----
src/backend/utils/adt/ruleutils.c | 37 ++++++++++++---------
src/test/regress/expected/rowtypes.out | 60 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/rowtypes.sql | 25 ++++++++++++++
4 files changed, 120 insertions(+), 22 deletions(-)