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: https://postgr.es/m/18077-b9db97c6e0ab45d8@postgresql.org

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d29812c0c6cd58794cb1c0eb1603a17a1ceb0d0b

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(-)