Add -Wimplicit-fallthrough to CFLAGS and CXXFLAGS

Use it at level 4, a bit more restrictive than the default level, and
tweak our commanding comments to FALLTHROUGH.

(However, leave zic.c alone, since it's external code; to avoid the
warnings that would appear there, change CFLAGS for that file in the
Makefile.)

Author: Julien Rouhaud <rjuju123@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20200412081825.qyo5vwwco3fv4gdo@nol
Discussion: https://postgr.es/m/flat/E1fDenm-0000C8-IJ@gemulon.postgresql.org

src/backend/replication/logical/reorderbuffer.c

@@ -1589,7 +1589,7 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 					change = specinsert;
 					change->action = REORDER_BUFFER_CHANGE_INSERT;
 
-					/* intentionally fall through */
+					/* FALLTHROUGH */
 				case REORDER_BUFFER_CHANGE_INSERT:
 				case REORDER_BUFFER_CHANGE_UPDATE:
 				case REORDER_BUFFER_CHANGE_DELETE:

src/backend/replication/walreceiver.c

@@ -215,7 +215,7 @@ WalReceiverMain(void)
 		case WALRCV_STOPPING:
 			/* If we've already been requested to stop, don't start up. */
 			walrcv->walRcvState = WALRCV_STOPPED;
-			/* fall through */
+			/* FALLTHROUGH */
 
 		case WALRCV_STOPPED:
 			SpinLockRelease(&walrcv->mutex);

src/backend/replication/walreceiverfuncs.c

@@ -183,7 +183,7 @@ ShutdownWalRcv(void)
 		case WALRCV_WAITING:
 		case WALRCV_RESTARTING:
 			walrcv->walRcvState = WALRCV_STOPPING;
-			/* fall through */
+			/* FALLTHROUGH */
 		case WALRCV_STOPPING:
 			walrcvpid = walrcv->pid;
 			break;