Move materialized views' is-populated status into their pg_class entries.

Previously this state was represented by whether the view's disk file had
zero or nonzero size, which is problematic for numerous reasons, since it's
breaking a fundamental assumption about heap storage.  This was done to
allow unlogged matviews to revert to unpopulated status after a crash
despite our lack of any ability to update catalog entries post-crash.
However, this poses enough risk of future problems that it seems better to
not support unlogged matviews until we can find another way.  Accordingly,
revert that choice as well as a number of existing kluges forced by it
in favor of creating a pg_class.relispopulated flag column.

src/backend/commands/cluster.c

@@ -30,7 +30,6 @@
 #include "catalog/objectaccess.h"
 #include "catalog/toasting.h"
 #include "commands/cluster.h"
-#include "commands/matview.h"
 #include "commands/tablecmds.h"
 #include "commands/vacuum.h"
 #include "miscadmin.h"
@@ -388,7 +387,7 @@ cluster_rel(Oid tableOid, Oid indexOid, bool recheck, bool verbose,
 	 * database.
 	 */
 	if (OldHeap->rd_rel->relkind == RELKIND_MATVIEW &&
-		!OldHeap->rd_ispopulated)
+		!RelationIsPopulated(OldHeap))
 	{
 		relation_close(OldHeap, AccessExclusiveLock);
 		return;
@@ -922,10 +921,6 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex,
 						get_namespace_name(RelationGetNamespace(OldHeap)),
 						RelationGetRelationName(OldHeap))));
 
-	if (OldHeap->rd_rel->relkind == RELKIND_MATVIEW)
-		/* Make sure the heap looks good even if no rows are written. */
-		SetMatViewToPopulated(NewHeap);
-
 	/*
 	 * Scan through the OldHeap, either in OldIndex order or sequentially;
 	 * copy each tuple into the NewHeap, or transiently to the tuplesort