QPersistentModelIndex: fix UB (op< on unrelated pointers)

Pointers can only be legitimately compared with less-than (<) if they
point into the same array (or one past the end). This is decidedly not
the case for heap-allocated objects like
QPersistentModelIndexPrivates, so doing it is UB.

Fix by using std::less, which is guaranteed to be a total order, even
for unrelated pointer values.

Pick-to: 6.6 6.5 6.2 5.15
Change-Id: If04341b4b55784e7732782f3ae829f53b0ceab9c
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/itemmodels/qabstractitemmodel.cpp

@@ -19,6 +19,8 @@
 #include <qdatetime.h>
 #include <qloggingcategory.h>
 
+#include <functional>
+
 #include <limits.h>
 
 QT_BEGIN_NAMESPACE
@@ -402,7 +404,7 @@ bool QPersistentModelIndex::operator<(const QPersistentModelIndex &other) const
     if (d && other.d)
         return d->index < other.d->index;
 
-    return d < other.d;
+    return std::less<>{}(d, other.d);
 }
 
 /*!