on placeholder entries in view rule action query's range table

Per Alvaro's advice, forking this from [1].

In light of my proposed changes to decouple permission checking from
the range table on that thread (now committed as a61b1f7482), I had
also been posting a patch there to rethink commands/view.c's
editorializing of a view rule action query' range table to add the
placeholder RTEs for checking the permissions of the view relation
among other things.

That patch came to life after Tom's comment in the same thread, where
he wondered if we could do away with those placeholder entries [2] if
permission checking details were to go elsewhere.

All but very recent versions of the patch were simply removing the
function UpdateRangeTableOfViewParse() that added those entries, such
that a view rule's action query would be stored with only the RTEs of
the relations mentioned in the view's query, with no trace whatsoever
of the view relation. ApplyRetrieveRule() working with a given user
query on the view would add a placeholder entry for the view for the
purpose served by those no-longer-present placeholder RTEs in the rule
action query's range table. It would accomplish that by adding a copy
of the query's view RTE with appropriate permission details filled in
before converting the latter into a RTE_SUBQUERY entry. However, this
approach of not storing the placeholder in the stored rule would lead
to a whole lot of regression test output changes, because the stored
view queries of many regression tests involving views would now end up
with only 1 entry in the range table instead of 3, causing ruleutils.c
to no longer qualify the column names in the deparsed representation
of those queries appearing in those regression test expected outputs.

To avoid that churn (not sure if really a goal to strive for in this
case!), I thought it might be better to keep the OLD entry in the
stored action query while getting rid of the NEW entry. Other than
avoiding the regression test output churn, this also makes the changes
of ApplyRetrieveRule unnecessary. Actually, as I was addressing
Alvaro's comments on the now-committed patch, I was starting to get
concerned about the implications of the change in position of the view
relation RTE in the query's range table if ApplyRetrieveRule() adds
one from scratch instead of simply recycling the OLD entry from stored
rule action query, even though I could see that there are no
*user-visible* changes, especially after decoupling permission
checking from the range table.

Anyway, the attached patch implements this 2nd approach.

I'll add this to the January CF.

--
Thanks, Amit Langote
EDB: http://www.enterprisedb.com

[1] https://www.postgresql.org/message-id/CA+HiwqGjJDmUhDSfv-U2qhKJjt9ST7Xh9JXC_irsAQ1TAUsJYg@mail.gmail.com
[2] https://www.postgresql.org/message-id/697679.1625154303%40sss.pgh.pa.us