Avoid erroneous creation of QScopedValueRollback objects

Mark the class as [[nodiscard]] to ensure that instances are given names
and do not destruct (roll back) immediately. This avoids accidental code
like this:

    QScopedValueRollback<Foo>(bar, baz);

which rolls back instantly, when it should be

    QScopedValueRollback<Foo> blah(bar, baz);

which rolls back at the end of the scope.

Change-Id: I00269fe325b804078bd0a9d5058c941af7ba5597
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/tools/qscopedvaluerollback.h

@@ -45,7 +45,11 @@
 QT_BEGIN_NAMESPACE
 
 template <typename T>
-class QScopedValueRollback
+class
+#if QT_HAS_CPP_ATTRIBUTE(nodiscard) && __cplusplus >= 201703L
+[[nodiscard]]
+#endif
+QScopedValueRollback
 {
 public:
     explicit QScopedValueRollback(T &var)