Allocate DatumHashTable in ANALYZE memory context

Instead of allocating under TopMemoryContext, we allocate it under
VacAttrStats->anl_context now.

Reviewed-by: Ashwin Agrawal <aashwin@vmware.com>
(cherry picked from commit b88beeb)

src/backend/commands/analyzeutils.c

@@ -444,6 +444,7 @@ createDatumHashTable(unsigned int nEntries)
 	hash_ctl.entrysize = sizeof(MCVFreqEntry);
 	hash_ctl.hash = datumHashTableHash;
 	hash_ctl.match = datumHashTableMatch;
+	hash_ctl.hcxt = CurrentMemoryContext; /* VacAttrStats->anl_context */
 
 	return hash_create("DatumHashTable", nEntries, &hash_ctl,
 					   HASH_ELEM | HASH_FUNCTION | HASH_COMPARE);