Fix variable shadowing in procarray.c.

ProcArrayGroupClearXid function has a parameter named "proc",
but the same name was used for its local variables. This commit fixes
this variable shadowing, to improve code readability.

Back-patch to all supported versions, to make future back-patching
easy though this patch is classified as refactoring only.

Reported-by: Ranier Vilela
Author: Ranier Vilela, Aleksander Alekseev
https://postgr.es/m/CAEudQAqyoTZC670xWi6w-Oe2_Bk1bfu2JzXz6xRfiOUzm7xbyQ@mail.gmail.com

src/backend/storage/ipc/procarray.c

@@ -840,12 +840,12 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	/* Walk the list and clear all XIDs. */
 	while (nextidx != INVALID_PGPROCNO)
 	{
-		PGPROC	   *proc = &allProcs[nextidx];
+		PGPROC	   *nextproc = &allProcs[nextidx];
 
-		ProcArrayEndTransactionInternal(proc, proc->procArrayGroupMemberXid);
+		ProcArrayEndTransactionInternal(nextproc, nextproc->procArrayGroupMemberXid);
 
 		/* Move to next proc in list. */
-		nextidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
+		nextidx = pg_atomic_read_u32(&nextproc->procArrayGroupNext);
 	}
 
 	/* We're done with the lock now. */
@@ -860,18 +860,18 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	 */
 	while (wakeidx != INVALID_PGPROCNO)
 	{
-		PGPROC	   *proc = &allProcs[wakeidx];
+		PGPROC	   *nextproc = &allProcs[wakeidx];
 
-		wakeidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
-		pg_atomic_write_u32(&proc->procArrayGroupNext, INVALID_PGPROCNO);
+		wakeidx = pg_atomic_read_u32(&nextproc->procArrayGroupNext);
+		pg_atomic_write_u32(&nextproc->procArrayGroupNext, INVALID_PGPROCNO);
 
 		/* ensure all previous writes are visible before follower continues. */
 		pg_write_barrier();
 
-		proc->procArrayGroupMember = false;
+		nextproc->procArrayGroupMember = false;
 
-		if (proc != MyProc)
-			PGSemaphoreUnlock(proc->sem);
+		if (nextproc != MyProc)
+			PGSemaphoreUnlock(nextproc->sem);
 	}
 }