QThread: fix UB (invalid enum value) on Private::Priority

The Unix code stores an additional flag, ThreadPriorityResetFlag, in
the Policy enum, but ubsan does not approve:

  qthread_unix.cpp:303:30: runtime error: load of value 2147483648, which is not a valid value for type 'Priority'
  qthread_unix.cpp:304:75: runtime error: load of value 2147483648, which is not a valid value for type 'Priority'

Fix by making the variable of std::underlying_type_t<Priority>.

The masking and unmasking code can now be simplified, too.

In the Windows version, replace some switch targets with equivalent
ones to keep -Wswitch-like warnings, though I hasten to note that both
switches use a default case, so have anyway implicitly disabled said
warning.

Pick-to: 6.3 6.2 5.15
Change-Id: Ie4ea7d05e2928d2755ad12d36535197f85493191
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/thread/qthread_p.h

@@ -164,7 +164,7 @@ public:
     int returnCode;
 
     uint stackSize;
-    QThread::Priority priority;
+    std::underlying_type_t<QThread::Priority> priority;
 
 #ifdef Q_OS_UNIX
     QWaitCondition thread_done;

src/corelib/thread/qthread_unix.cpp

@@ -322,8 +322,8 @@ void *QThreadPrivate::start(void *arg)
             QMutexLocker locker(&thr->d_func()->mutex);
 
             // do we need to reset the thread priority?
-            if (qToUnderlying(thr->d_func()->priority) & ThreadPriorityResetFlag) {
-                thr->d_func()->setPriority(QThread::Priority(qToUnderlying(thr->d_func()->priority) & ~ThreadPriorityResetFlag));
+            if (thr->d_func()->priority & ThreadPriorityResetFlag) {
+                thr->d_func()->setPriority(QThread::Priority(thr->d_func()->priority & ~ThreadPriorityResetFlag));
             }
 
             // threadId is set in QThread::start()
@@ -703,7 +703,7 @@ void QThread::start(Priority priority)
                 // could not set scheduling hints, fallback to inheriting them
                 // we'll try again from inside the thread
                 pthread_attr_setinheritsched(&attr, PTHREAD_INHERIT_SCHED);
-                d->priority = Priority(qToUnderlying(priority) | ThreadPriorityResetFlag);
+                d->priority = qToUnderlying(priority) | ThreadPriorityResetFlag;
             }
             break;
         }

src/corelib/thread/qthread_win.cpp

@@ -457,7 +457,7 @@ void QThread::start(Priority priority)
 
     int prio;
     d->priority = priority;
-    switch (d->priority) {
+    switch (priority) {
     case IdlePriority:
         prio = THREAD_PRIORITY_IDLE;
         break;
@@ -584,7 +584,7 @@ void QThreadPrivate::setPriority(QThread::Priority threadPriority)
 
     int prio;
     priority = threadPriority;
-    switch (priority) {
+    switch (threadPriority) {
     case QThread::IdlePriority:
         prio = THREAD_PRIORITY_IDLE;
         break;