QDebugStateSaver: mark ctors [[nodiscard]]

It's a RAII class, and RAII classes should have only [[nodiscard]]
constructors.

Pick-to: 6.6
Task-number: QTBUG-104164
Change-Id: Ia83fa003677a839734208b12bde2c6287c1b79a3
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/io/qdebug.h

@@ -214,7 +214,7 @@ class QDebugStateSaverPrivate;
 class QDebugStateSaver
 {
 public:
-    Q_CORE_EXPORT
+    Q_NODISCARD_CTOR Q_CORE_EXPORT
     QDebugStateSaver(QDebug &dbg);
     Q_CORE_EXPORT
     ~QDebugStateSaver();