Fix rvalue overload of qobject_pointer_cast for GCC 9.3

The code assumed that any C++ implementation would implement the
resolution for LWG2996 in C++20 mode. While that may be the case in
the future, the current state in GCC 9.3 as shipped in Ubuntu 20.04
LTS is that it doesn't, which leads to tst_qsharedpointer fail there.

Fix by using the safe version of std::move, std::exchange, which
guarantees the state of the src object, no matter what the callee
does.

Pick-to: 6.2 6.1 5.15
Change-Id: Icc39b527df4d3a7b398ff2b44bcbdf9082b81f2f
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/tools/qsharedpointer_impl.h

@@ -931,15 +931,11 @@ std::shared_ptr<X> qobject_pointer_cast(std::shared_ptr<T> &&src)
     using element_type = typename std::shared_ptr<X>::element_type;
     auto castResult = qobject_cast<element_type *>(src.get());
     if (castResult) {
-        auto result = std::shared_ptr<X>(std::move(src), castResult);
-#if __cplusplus <= 201703L
         // C++2a's move aliasing constructor will leave src empty.
         // Before C++2a we don't really know if the compiler has support for it.
         // The move aliasing constructor is the resolution for LWG2996,
         // which does not impose a feature-testing macro. So: clear src.
-        src.reset();
-#endif
-        return result;
+        return std::shared_ptr<X>(std::exchange(src, nullptr), castResult);
     }
     return std::shared_ptr<X>();
 }