Abuse const_cast in QStringBuilder to get a little more performance

Without this const_cast, the call to QString::data() or
QByteArray::data() will cause a call to detach(), which is totally
unnecessary since we've just allocated data a couple of lines before.
Since we know we're the owner of the only reference, we can skip the
detach attempt.

Change-Id: If40f66100f85cc9b405025f21c562828ead23475
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qstringbuilder.h

@@ -112,7 +112,9 @@ private:
         const uint len = QConcatenable< QStringBuilder<A, B> >::size(*this);
         T s(len, Qt::Uninitialized);
 
-        typename T::iterator d = s.data();
+        // we abuse const_cast / constData here because we know we've just
+        // allocated the data and we're the only reference count
+        typename T::iterator d = const_cast<typename T::iterator>(s.constData());
         typename T::const_iterator const start = d;
         QConcatenable< QStringBuilder<A, B> >::appendTo(*this, d);