Use fallthrough comments to suppress -Wimplicit-fallthrough comments.

src/backend/distributed/relay/relay_event_utility.c

@@ -164,9 +164,8 @@ RelayEventExtendNames(Node *parseTree, char *schemaName, uint64 shardId)
 			 * performed on CreateStmt should be done here too, we simply *fall
 			 * through* to avoid code repetition.
 			 */
-			__attribute__((fallthrough));
 		}
-
+		/* fallthrough */
 		case T_CreateStmt:
 		{
 			CreateStmt *createStmt = (CreateStmt *) parseTree;

src/backend/distributed/utils/ruleutils_96.c

@@ -4448,10 +4448,8 @@ isSimpleNode(Node *node, Node *parentNode, int prettyFlags)
 					return false;
 				}
 				/* else do the same stuff as for T_SubLink et al. */
-				/* FALL THROUGH */
-				__attribute__((fallthrough));
 			}
-
+		/* fallthrough */
 		case T_SubLink:
 		case T_NullTest:
 		case T_BooleanTest: