pgsql: Print the correct aliases for DML target tables in ruleutils.

Print the correct aliases for DML target tables in ruleutils.

ruleutils.c blindly printed the user-given alias (or nothing if there
hadn't been one) for the target table of INSERT/UPDATE/DELETE queries.
That works a large percentage of the time, but not always: for queries
appearing in WITH, it's possible that we chose a different alias to
avoid conflict with outer-scope names.  Since the chosen alias would
be used in any Var references to the target table, this'd lead to an
inconsistent printout with consequences such as dump/restore failures.

The correct logic for printing (or not) a relation alias was embedded
in get_from_clause_item.  Factor it out to a separate function so that
we don't need a jointree node to use it.  (Only a limited part of that
function can be reached from these new call sites, but this seems like
the cleanest non-duplicative factorization.)

In passing, I got rid of a redundant "\d+ rules_src" step in rules.sql.

Initial report from Jonathan Katz; thanks to Vignesh C for analysis.
This has been broken for a long time, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/e947fa21-24b2-f922-375a-d4f763ef3e4b@postgresql.org
Discussion: https://postgr.es/m/CALDaNm1MMntjmT_NJGp-Z=xbF02qHGAyuSHfYHias3TqQbPF2w@mail.gmail.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/df931e9ab35bae8902035eac60e0edcbd8db8b3d

Modified Files
--------------
src/backend/utils/adt/ruleutils.c   | 145 +++++++++++++++++++++---------------
src/test/regress/expected/rules.out |  47 +++++++-----
src/test/regress/sql/rules.sql      |  13 +++-
3 files changed, 126 insertions(+), 79 deletions(-)