Fix misuse of "const" qualifier.

"const foo *" is quite different from "foo * const".
This code was evidently trying to avoid casting away
const from the arguments, but entirely failed to do so.

Per study of some buildfarm warnings from anole
(which unfortunately are mostly ignorable, since it
seems not to understand "restrict" very well).
I'm surprised though that nothing else has complained.

Author: tglsfdc

src/backend/partitioning/partbounds.c

@@ -3793,8 +3793,8 @@ partition_hash_bsearch(PartitionBoundInfo boundinfo,
 static int32
 qsort_partition_hbound_cmp(const void *a, const void *b)
 {
-	PartitionHashBound *const h1 = (PartitionHashBound *const) a;
-	PartitionHashBound *const h2 = (PartitionHashBound *const) b;
+	const PartitionHashBound *h1 = (const PartitionHashBound *) a;
+	const PartitionHashBound *h2 = (const PartitionHashBound *) b;
 
 	return partition_hbound_cmp(h1->modulus, h1->remainder,
 								h2->modulus, h2->remainder);
@@ -3808,8 +3808,8 @@ qsort_partition_hbound_cmp(const void *a, const void *b)
 static int32
 qsort_partition_list_value_cmp(const void *a, const void *b, void *arg)
 {
-	Datum		val1 = ((PartitionListValue *const) a)->value,
-				val2 = ((PartitionListValue *const) b)->value;
+	Datum		val1 = ((const PartitionListValue *) a)->value,
+				val2 = ((const PartitionListValue *) b)->value;
 	PartitionKey key = (PartitionKey) arg;
 
 	return DatumGetInt32(FunctionCall2Coll(&key->partsupfunc[0],