Explicit switch/case fall-throughs to avoid compiler warnings.

GCC 7 added `-Wimplicit-fallthrough` to warn for not explicitly specified switch/case fall-throughs.

According to https://gcc.gnu.org/gcc-7/changes.html, to suppress that warning we could either use `__attribute__(fallthrough)`, which didn't seem to work for earlier GCC versions, or a `/* fallthrough */` comment just before the following `case`.

Previously Citus code had the fall-through comments inside the brackets, which didn't seem to suppress the warning. Putting a `/* fallthrough */` comment outside the brackets and right before the `case` fixes the problem.

src/backend/distributed/relay/relay_event_utility.c

@@ -166,6 +166,7 @@ RelayEventExtendNames(Node *parseTree, char *schemaName, uint64 shardId)
 			 */
 		}
 
+		/* fallthrough */
 		case T_CreateStmt:
 		{
 			CreateStmt *createStmt = (CreateStmt *) parseTree;

src/backend/distributed/utils/ruleutils_96.c

@@ -4448,9 +4448,9 @@ isSimpleNode(Node *node, Node *parentNode, int prettyFlags)
 					return false;
 				}
 				/* else do the same stuff as for T_SubLink et al. */
-				/* FALL THROUGH */
 			}
 
+		/* fallthrough */
 		case T_SubLink:
 		case T_NullTest:
 		case T_BooleanTest: