VxWorks seems to have a problem with std::optional

When trying to use std::optional instead of QScopedPointer with a QSignalSpy payload, the VxWorks platform segfaulted:

   QWARN  : tst_QAbstractAnimation::stateBinding() Trying to construct an instance of an invalid type, type id: -2125884168
   QWARN  : tst_QAbstractAnimation::stateBinding() Trying to construct an instance of an invalid type, type id: -2125884172
   QWARN  : tst_QAbstractAnimation::stateBinding() Trying to construct an instance of an invalid type, type id: -2125884024
   Received signal 11 (SIGSEGV), code 2, for address 0x0000000d

(cf. integrations and prechecks of PS1..2 of https://codereview.qt-project.org/c/qt/qtbase/+/612065)

std::optional is used in many Qt APIs already. If VxWorks has a problem in its implementation, we need to

• get it fixed upstream
• or, failing that, create rules for safe std::optional use

Once we understand the issue, we should follow-up on https://codereview.qt-project.org/c/qt/qtbase/+/612065 to get rid of the Q_OS_VXWORKS.