Silence complaints about leaks in PlanCacheComputeResultDesc.

Silence complaints about leaks in PlanCacheComputeResultDesc.

CompleteCachedPlan intentionally doesn't worry about small
leaks from PlanCacheComputeResultDesc.  However, Valgrind
knows nothing of engineering tradeoffs and complains anyway.
Silence it by doing things the hard way if USE_VALGRIND.

I don't really love this patch, because it makes the handling
of plansource->resultDesc different from the handling of query
dependencies and search_path just above, which likewise are willing
to accept small leaks into the cached plan's context.  However,
those cases aren't provoking Valgrind complaints.  (Perhaps in a
CLOBBER_CACHE_ALWAYS build, they would?)  For the moment, this
makes the src/pl/plpgsql tests leak-free according to Valgrind.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/285483.1746756246@sss.pgh.pa.us