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/createas.c

@@ -359,10 +359,6 @@ intorel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
 	 */
 	intoRelationDesc = heap_open(intoRelationId, AccessExclusiveLock);
 
-	if (is_matview && !into->skipData)
-		/* Make sure the heap looks good even if no rows are written. */
-		SetMatViewToPopulated(intoRelationDesc);
-
 	/*
 	 * Check INSERT permission on the constructed table.
 	 *
@@ -381,6 +377,13 @@ intorel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
 
 	ExecCheckRTPerms(list_make1(rte), true);
 
+	/*
+	 * Tentatively mark the target as populated, if it's a matview and we're
+	 * going to fill it; otherwise, no change needed.
+	 */
+	if (is_matview && !into->skipData)
+		SetMatViewPopulatedState(intoRelationDesc, true);
+
 	/*
 	 * Fill private fields of myState for use by later routines
 	 */