MDEV-20425: Fix -Wimplicit-fallthrough

With --skip-debug-assert, DBUG_ASSERT(false) will allow execution to
continue. Hence, we will need /* fall through */ after them.

Some DBUG_ASSERT(0) were replaced by break; when the switch () statement
was followed by DBUG_ASSERT(0).

sql/sql_lex.cc

@@ -2911,6 +2911,7 @@ void st_select_lex_unit::print(String *str, enum_query_type query_type)
       {
       default:
         DBUG_ASSERT(0);
+        /* fall through */
       case UNION_TYPE:
         str->append(STRING_WITH_LEN(" union "));
         if (union_all)