Add a reverse-translation column number array to struct AppendRelInfo.

This provides for cheaper mapping of child columns back to parent
columns.  The one existing use-case in examine_simple_variable()
would hardly justify this by itself; but an upcoming bug fix will
make use of this array in a mainstream code path, and it seems
likely that we'll find other uses for it as we continue to build
out the partitioning infrastructure.

Discussion: https://postgr.es/m/12424.1575168015@sss.pgh.pa.us

src/backend/nodes/nodeFuncs.c

@@ -3103,6 +3103,7 @@ expression_tree_mutator(Node *node,
 
 				FLATCOPY(newnode, appinfo, AppendRelInfo);
 				MUTATE(newnode->translated_vars, appinfo->translated_vars, List *);
+				/* Assume nothing need be done with parent_colnos[] */
 				return (Node *) newnode;
 			}
 			break;