Consider collation when proving subquery uniqueness

Consider collation when proving subquery uniqueness

rel_is_distinct_for()'s RTE_SUBQUERY branch passed only the equality
operator from each join clause to query_is_distinct_for(), discarding
the operator's input collation.  query_is_distinct_for() then verified
opfamily compatibility but never checked collations, so a DISTINCT /
GROUP BY / set-op operating under one collation was trusted to prove
uniqueness for a comparison performed under an unrelated collation.
As with the recent fix in relation_has_unique_index_for(), this is
unsound for nondeterministic collations and yields wrong query results
in any optimization that consumes the proof.

Fix by carrying each clause's operator input collation into
query_is_distinct_for() and validating it at every check-site against
the subquery target expression's collation.

Back-patch to all supported branches.  query_is_distinct_for() is
declared in an installed header, so on stable branches the existing
two-list signature is retained as a thin wrapper that forwards to a
new collation-aware entry point; external callers continue to receive
the historical collation-blind answer.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAMbWs4_XUUSTyzCaRjUeeahWNqi=8ZOA5Q4coi8zUVEDSBkM6A@mail.gmail.com
Backpatch-through: 14