Fix setrefs.c's failure to do expression processing on prune steps.

Fix setrefs.c's failure to do expression processing on prune steps.

We should run the expression subtrees of PartitionedRelPruneInfo
structs through fix_scan_expr.  Failure to do so means that
AlternativeSubPlans within those expressions won't be cleaned up
properly, resulting in "unrecognized node type" errors since v14.

It seems fairly likely that at least some of the other steps done
by fix_scan_expr are important here as well, resulting in as-yet-
undetected bugs.  Therefore, I've chosen to back-patch this to
all supported branches including v13, even though the known
symptom doesn't manifest in v13.

Per bug #18778 from Alexander Lakhin.

Discussion: http://postgr.es/m/18778-24cd399df6c806af@postgresql.org