C++20 removed std::result_of

Description:
In /std:c+17 mode, this triggers a deprecation warning in recent versions of MSVC. In /std:c+latest mode, now that microsoft/STL#380 has been merged, this will trigger an error in VS 2019 16.6 Preview 2.
Reproduce steps:
1. git clone git://code.qt.io/qt/qt5.git D:\QT5\src
2. cd /d D:\QT5\src
3. git checkout dev
4. perl init-repository
5. set CL=/D_ENABLE_EXTENDED_ALIGNED_STORAGE
6. configure -developer-build -opensource -release -nomake examples -nomake tests -skip qtwebengine -confirm-license 2>&1
 
ErrorMessage:
F:\gitP\qt\qt5\qtbase\include\QtCore\../../src/corelib/kernel/qobjectdefs.h(535): error C2039: 'result_of': is not a member of 'std'