Short live q20::construct_at()!

Move the helper from qsystemsemaphore.cpp to q20memory.h
to prevent clashes in CMake Unity (Jumbo) builds.

Manual conflict resolutions:
  - file-local construct_at doesn't exist in 6.5

Task-number: QTBUG-109394
Change-Id: Id0127af1f0d51c87a5887090cc90ab232eff8093
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit 72c2cdbc572f8b8b45a57a451e2bc19bb1c53b0c)
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/CMakeLists.txt

@@ -103,6 +103,7 @@ qt_internal_add_module(Core
         global/q20algorithm.h
         global/q20functional.h
         global/q20iterator.h
+        global/q20memory.h
         global/q20type_traits.h
         global/q23functional.h
         global/qxpfunctional.h

src/corelib/global/q20memory.h

@@ -0,0 +1,46 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#ifndef Q20MEMORY_H
+#define Q20MEMORY_H
+
+#include <QtCore/qxptype_traits.h>
+
+#include <memory>
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API. Types and functions defined in this
+// file can reliably be replaced by their std counterparts, once available.
+// You may use these definitions in your own code, but be aware that we
+// will remove them once Qt depends on the C++ version that supports
+// them in namespace std. There will be NO deprecation warning, the
+// definitions will JUST go away.
+//
+// If you can't agree to these terms, don't use these definitions!
+//
+// We mean it.
+//
+
+QT_BEGIN_NAMESPACE
+
+namespace q20 {
+
+#if __cplusplus >= 202002L
+using std::construct_at;
+#else
+template <typename T,
+          typename... Args,
+          typename Enable = std::void_t<decltype(::new (std::declval<void *>()) T(std::declval<Args>()...))> >
+         static void construct_at(T *ptr, Args && ... args)
+{
+    ::new (const_cast<void*>(static_cast<const volatile void*>(ptr))) T(std::forward<Args>(args)...);
+}
+#endif
+} // namespace q20
+
+QT_END_NAMESPACE
+
+#endif /* Q20MEMORY_H */