QGlobalStatic: invert the order of destruction and setting the guard

This is how the old implementation did it: the Type member was a member
of Holder, but the guard was set to Destroyed in the HolderBase
destructor, which ran after. I find the way I implemented in
commit81a31beeb25eaf14d5c5f42fe26aa49d6ef29bf8 to be more natural, but
it caused regressions at runtime for code that attempted to reenter the
global static on destruction.

Not unit-tested because I don't know if we want to keep this forever.

Pick-to: 6.3
Fixes: QTBUG-99192
Change-Id: Ib42b3adc93bf4d43bd55fffd16c09d7f835d121e
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/global/qglobalstatic.h

@@ -76,9 +76,9 @@ template <typename QGS> union Holder
 
     ~Holder()
     {
-        guard.storeRelaxed(QtGlobalStatic::Destroyed);
-        std::atomic_thread_fence(std::memory_order_acquire); // avoid mixing stores to guard and *pointer()
         pointer()->~PlainType();
+        std::atomic_thread_fence(std::memory_order_acquire); // avoid mixing stores to guard and *pointer()
+        guard.storeRelaxed(QtGlobalStatic::Destroyed);
     }
 
     PlainType *pointer() noexcept