Fix some confusing uses of const

There are a few places where we have

    typedef struct FooData { ... } FooData;
    typedef FooData *Foo;

and then function declarations with

    bar(const Foo x)

which isn't incorrect but probably meant

    bar(const FooData *x)

meaning that the thing x points to is immutable, not x itself.

This patch makes those changes where appropriate.  In one
case (execGrouping.c), the thing being pointed to was not immutable,
so in that case remove the const altogether, to avoid further
confusion.

Co-authored-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAEoWx2m2E0xE8Kvbkv31ULh_E%2B5zph-WA_bEdv3UR9CLhw%2B3vg%40mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAEoWx2kTDz%3Db6T2xHX78vy_B_osDeCC5dcTCi9eG0vXHp5QpdQ%40mail.gmail.com

src/backend/access/gin/ginget.c

@@ -489,7 +489,7 @@ restartScanEntry:
 static int
 entryIndexByFrequencyCmp(const void *a1, const void *a2, void *arg)
 {
-	const GinScanKey key = (const GinScanKey) arg;
+	const GinScanKeyData *key = arg;
 	int			i1 = *(const int *) a1;
 	int			i2 = *(const int *) a2;
 	uint32		n1 = key->scanEntry[i1]->predictNumberResult;

src/backend/access/gin/ginpostinglist.c

@@ -84,7 +84,7 @@
 #define MaxBytesPerInteger				7
 
 static inline uint64
-itemptr_to_uint64(const ItemPointer iptr)
+itemptr_to_uint64(const ItemPointerData *iptr)
 {
 	uint64		val;
 
@@ -194,7 +194,7 @@ decode_varbyte(unsigned char **ptr)
  * byte at the end, if any, is zero.
  */
 GinPostingList *
-ginCompressPostingList(const ItemPointer ipd, int nipd, int maxsize,
+ginCompressPostingList(const ItemPointerData *ipd, int nipd, int maxsize,
 					   int *nwritten)
 {
 	uint64		prev;