pgsql: Add a reverse-translation column number array to struct AppendRe
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.