Short live qExchange()!

This is a 1:1 replacement for std::exchange, and should be removed once
Qt fully depends on C++14. It's too versatile a tool to miss it, so
provide a copy.

[ChangeLog][QtCore][QtGlobal] Added qExchange(), a drop-in for C++14's std::exchange()

Change-Id: I31c4f1141e7a99f99ea65eb36ddf9d68b7847337
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.cpp

@@ -3805,6 +3805,17 @@ bool qunsetenv(const char *varName)
     \snippet code/src_corelib_global_qglobal.cpp as-const-4
 */
 
+/*!
+    \fn template <typename T, typename U = T> T qExchange(T &obj, U &&newValue)
+    \relates <QtGlobal>
+    \since 5.14
+
+    Replaces the value of \a obj with \a newValue and returns the old value of \a obj.
+
+    This is Qt's implementation of std::exchange(). It differs from std::exchange()
+    only in that it is \c constexpr already in C++14, and available on all supported
+    compilers.
+*/
 /*!
     \macro QT_TR_NOOP(sourceText)
     \relates <QtGlobal>

src/corelib/global/qglobal.h

@@ -1008,6 +1008,15 @@ Q_DECL_CONSTEXPR typename std::add_const<T>::type &qAsConst(T &t) noexcept { ret
 template <typename T>
 void qAsConst(const T &&) = delete;
 
+// like std::exchange
+template <typename T, typename U = T>
+Q_DECL_RELAXED_CONSTEXPR T qExchange(T &t, U &&newValue)
+{
+    T old = std::move(t);
+    t = std::forward<U>(newValue);
+    return old;
+}
+
 #ifndef QT_NO_FOREACH
 
 namespace QtPrivate {