QScopedValueRollback: code tidies

Add constexpr + deploy std::exchange.

Change-Id: I4c5e6f5d35cd74c464667f633344d27c0656f0eb
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/corelib/tools/qscopedvaluerollback.h

@@ -52,23 +52,25 @@ class
 QScopedValueRollback
 {
 public:
-    explicit QScopedValueRollback(T &var)
+    explicit constexpr QScopedValueRollback(T &var)
         : varRef(var), oldValue(var)
     {
     }
 
-    explicit QScopedValueRollback(T &var, T value)
-        : varRef(var), oldValue(std::move(var))
+    explicit constexpr QScopedValueRollback(T &var, T value)
+        : varRef(var), oldValue(qExchange(var, std::move(value)))
     {
-        varRef = std::move(value);
     }
 
+#if __cpp_constexpr >= 201907L
+    constexpr
+#endif
     ~QScopedValueRollback()
     {
         varRef = std::move(oldValue);
     }
 
-    void commit()
+    constexpr void commit()
     {
         oldValue = varRef;
     }