Short live q20::exchange()!

We have weaned ourselves off of qExchange(), which, however, has C++23
semantics (is constexpr and noexcept), while we only require C++17 atm.
At the same time, we have more and more uses of a constexpr exchange(),
iow: the C++20 extension.

We have the qNN namespaces for this, so let's use it: Add
q20::exchange(), with C++20 semantics (constexpr, not noexcept), and
use it to port the only remaining qExchange() user (since ported to
2×std::move() to make the header compatible with QT_NO_QEXCHANGE),
QScopedValueRollback.

No user requires the C++23 noexcept on the function (compilers will
figure it out, because the std::exchange is fully inline), and we
can't provide a q23::exchange() even if we needed to, because the
author of P2401 forgot to ask to update the value of
__cpp_lib_exchange_function...

Fixes: QTBUG-133038
Pick-to: 6.8
Change-Id: I003df445001d1c5ab02402119c5e5106aa156263
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit b5ed7fb203335cef7b7b4f70f4130b07166cecf2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/q20utility.h

@@ -118,6 +118,21 @@ constexpr bool in_range(T t) noexcept
 #endif // __cpp_lib_integer_comparison_functions
 } // namespace q20
 
+// like C++20 std::exchange (ie. constexpr, not yet noexcept)
+namespace q20 {
+#ifdef __cpp_lib_constexpr_algorithms
+using std::exchange;
+#else
+template <typename T, typename U = T>
+constexpr T exchange(T& obj, U&& newValue)
+{
+    T old = std::move(obj);
+    obj = std::forward<U>(newValue);
+    return old;
+}
+#endif
+}
+
 QT_END_NAMESPACE
 
 #endif /* Q20UTILITY_H */

src/corelib/tools/qscopedvaluerollback.h

@@ -6,6 +6,8 @@
 
 #include <QtCore/qglobal.h>
 
+#include <QtCore/q20utility.h>
+
 QT_BEGIN_NAMESPACE
 
 template <typename T>
@@ -20,9 +22,8 @@ public:
 
     Q_NODISCARD_CTOR
     explicit constexpr QScopedValueRollback(T &var, T value)
-        : varRef(var), oldValue(std::move(var)) // ### C++20: std::exchange(var, std::move(value))
+        : varRef(var), oldValue(q20::exchange(var, std::move(value)))
     {
-        var = std::move(value);
     }
 
     Q_DECL_CONSTEXPR_DTOR