Fix additional fallthrough warnings from clang

Clang warns if falling through to a case or default label that is
immediately followed by break, but GCC does
not (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432).  (MSVC also
warns about the equivalent code in C++.)

This is in preparation for enabling fallthrough warnings on Clang.

Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/76a8efcd-925a-4eaf-bdd1-d972cd1a32ff%40eisentraut.org

contrib/isn/isn.c

@@ -837,6 +837,7 @@ string2ean(const char *str, struct Node *escontext, ean13 *result,
 		case UPC:
 			buf[2] = '0';
 			valid = (valid && ((rcheck = checkdig(buf + 2, 13)) == check || magic));
+			break;
 		default:
 			break;
 	}

contrib/pg_overexplain/pg_overexplain.c

@@ -258,6 +258,7 @@ overexplain_per_node_hook(PlanState *planstate, List *ancestors,
 					overexplain_bitmapset("RTIs",
 										  ((Result *) plan)->relids,
 										  es);
+				break;
 			default:
 				break;
 		}

src/backend/access/transam/xlogrecovery.c

@@ -1895,6 +1895,7 @@ PerformWalRecovery(void)
 					recoveryPausesHere(true);
 
 					/* drop into promote */
+					pg_fallthrough;
 
 				case RECOVERY_TARGET_ACTION_PROMOTE:
 					break;

src/backend/replication/logical/decode.c

@@ -430,9 +430,8 @@ heap2_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 
 				xlrec = (xl_heap_new_cid *) XLogRecGetData(buf->record);
 				SnapBuildProcessNewCid(builder, xid, buf->origptr, xlrec);
-
-				break;
 			}
+			break;
 		case XLOG_HEAP2_REWRITE:
 
 			/*

src/backend/tcop/dest.c

@@ -180,6 +180,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 			len = BuildQueryCompletionString(completionTag, qc,
 											 force_undecorated_output);
 			pq_putmessage(PqMsg_CommandComplete, completionTag, len + 1);
+			break;
 
 		case DestNone:
 		case DestDebug:

src/backend/utils/mb/conversion_procs/euc_tw_and_big5/big5.c

@@ -370,6 +370,7 @@ CNStoBIG5(unsigned short cns, unsigned char lc)
 				if (b1c4[i][1] == cns)
 					return b1c4[i][0];
 			}
+			break;
 		default:
 			break;
 	}

src/pl/plpgsql/src/pl_comp.c

@@ -2333,6 +2333,7 @@ plpgsql_add_initdatums(int **varnos)
 					case PLPGSQL_DTYPE_VAR:
 					case PLPGSQL_DTYPE_REC:
 						(*varnos)[n++] = plpgsql_Datums[i]->dno;
+						break;
 
 					default:
 						break;

src/tools/pg_bsd_indent/indent.c

@@ -296,6 +296,7 @@ main(int argc, char **argv)
 		 * done earlier.
 		 */
 		force_nl = false;
+		break;
 	    case form_feed:
 		break;
 	    case comment: