Defend against leaks into RelationBuildPartitionDesc.

Defend against leaks into RelationBuildPartitionDesc.

In normal builds, this isn't very important, because the leaks go
into fairly short-lived contexts, but under CLOBBER_CACHE_ALWAYS,
this can result in leaking hundreds of megabytes into MessageContext,
which probably explains recent failures on hyrax.

This may or may not be the best long-term strategy for dealing
with this leak, but we can change it later if we come up with
something better.  For now, do this to make the buildfarm green
again (hopefully).  Commit 898e5e3290a72d288923260143930fb32036c00c
seems to have exacerbated this problem for reasons that are not
quite clear, but I don't believe it's actually the cause.

Discussion: http://postgr.es/m/CA+TgmoY3bRmGB6-DUnoVy5fJoreiBJ43rwMrQRCdPXuKt4Ykaw@mail.gmail.com