Backport a super-simplified version of #6792, fixing that #exists? can produce invalid SQL: "SELECT DISTINCT DISTINCT"

To recap, the combination of a :uniq => true association and the #distinct call
in #construct_limited_ids_condition combine to create invalid SQL, because
we're explicitly selecting DISTINCT, and also sending #distinct on to AREL,
via the relation#distinct_value.

Where #6792 was the forever fix, this is the minimal fix. Instead of
properly indicating the distinctness of the query through #uniq_value alone,
we use the literal DISTINCT select statement produced by #distinct and
set #uniq_value to always be falsey

Happy to backport the full #6792 change, but have the sense this is a simpler
intervention for the 3.2 line.