Don't use partial unique indexes for unique proofs in the planner

 Don't use partial unique indexes for unique proofs in the planner
 
 Here we adjust relation_has_unique_index_for() so that it no longer makes
 use of partial unique indexes as uniqueness proofs.  It is incorrect to
 use these as the predicates used by check_index_predicates() to set
 predOK makes use of not only baserestrictinfo quals as proofs, but also
 qual from join conditions.  For relation_has_unique_index_for()'s case, we
 need to know the relation is unique for a given set of columns before any
 joins are evaluated, so if predOK was only set to true due to some join
 qual, then it's unsafe to use such indexes in
 relation_has_unique_index_for().  The final plan may not even make use
 of that index, which could result in reading tuples that are not as
 unique as the planner previously expected them to be.
 
 Bug: #17975
 Reported-by: Tor Erik Linnerud
 Backpatch-through: 11, all supported versions
 Discussion: https://postgr.es/m/17975-98a90c156f25c952%40postgresql.org