Silence bogus GCC 7 warnings -Wimplicit-fallthrough

Do not silence uncertain cases, or fix any bugs.

The only functional change should be that ha_federated::extra()
is not calling DBUG_PRINT to report an unhandled case for
HA_EXTRA_PREPARE_FOR_DROP.

storage/connect/xobject.cpp

@@ -405,7 +405,7 @@ bool STRING::Append_quoted(PSZ s)
       case '\r':
       case '\b':
       case '\f': b |= Append('\\');
-        // passthru
+        // fall through
       default:
         b |= Append(*p);
         break;