QXmlStreamStringRef: mark as shared-not-movable-until-qt6

Requires adding member swap. Member-swap is also
necessary to fix the deleted move-assignment operator
of this class, on which QXmlStreamAttribute heavily
relies for its own move assignment operator.

That's why it's not proposed for dev, but 5.6.

Change-Id: Id7d0823d44fc6e55ada7c096ae95444f6bb50963
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/corelib/xml/qxmlstream.cpp

@@ -2622,6 +2622,13 @@ QXmlStreamEntityDeclaration::~QXmlStreamEntityDeclaration()
 {
 }
 
+/*! \fn QXmlStreamStringRef::swap(QXmlStreamStringRef &other)
+    \since 5.6
+
+    Swaps this string reference's contents with \a other.
+    This function is very fast and never fails.
+*/
+
 /*! \fn QStringRef QXmlStreamEntityDeclaration::name() const
 
 Returns the entity name.

src/corelib/xml/qxmlstream.h

@@ -54,12 +54,21 @@ public:
         :m_string(aString.string()?*aString.string():QString()), m_position(aString.position()), m_size(aString.size()){}
     inline QXmlStreamStringRef(const QString &aString):m_string(aString), m_position(0), m_size(aString.size()){}
     inline ~QXmlStreamStringRef(){}
+
+    void swap(QXmlStreamStringRef &other) Q_DECL_NOTHROW
+    {
+        qSwap(m_string, other.m_string);
+        qSwap(m_position, other.m_position);
+        qSwap(m_size, other.m_size);
+    }
+
     inline void clear() { m_string.clear(); m_position = m_size = 0; }
     inline operator QStringRef() const { return QStringRef(&m_string, m_position, m_size); }
     inline const QString *string() const { return &m_string; }
     inline int position() const { return m_position; }
     inline int size() const { return m_size; }
 };
+Q_DECLARE_SHARED_NOT_MOVABLE_UNTIL_QT6(QXmlStreamStringRef)
 
 
 class QXmlStreamReaderPrivate;