QVariant: move and swap in the move assignment operator

And not pure swap. As per policy, QVariant must leave the rhs empty.

Change-Id: I2d5e0f584c4d4fffd05a0a5bfae27ddcb72430e9
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/kernel/qvariant.h

@@ -271,7 +271,7 @@ class Q_CORE_EXPORT QVariant
     inline QVariant(QVariant &&other) noexcept : d(other.d)
     { other.d = Private(); }
     inline QVariant &operator=(QVariant &&other) noexcept
-    { qSwap(d, other.d); return *this; }
+    { QVariant moved(std::move(other)); swap(moved); return *this; }
 
     inline void swap(QVariant &other) noexcept { qSwap(d, other.d); }