Mark QtPrivate::StreamStateSaver's ctor as [[nodiscard]]

As a RAII class, QtPrivate::StreamStateSaver's ctors should be marked
as [[nodiscard]].

Amends 3c93286f08a80b6e1821d7d63d361742b25c6578, but not picking all
the way back since 959800f6de137f6a77c7d5a2741a5bae0638cbd9
(Q_NODISCARD_CTOR) was only added in 6.6.

QUIP: 19
Pick-to: 6.9 6.8
Change-Id: If39fd79d21c52d60481973a416ae5f7da1858f7a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qdatastream.h

@@ -270,6 +270,7 @@ class StreamStateSaver
 {
     Q_DISABLE_COPY_MOVE(StreamStateSaver)
 public:
+    Q_NODISCARD_CTOR
     explicit StreamStateSaver(QDataStream *s) : stream(s), oldStatus(s->status())
     {
         if (!stream->isDeviceTransactionStarted())