QTimer: make both defaultTypeFor() methods constexpr and noexcept

Also de-duplicate the code by using defaultTypeFor() in singleShot(),
this way the comment and the calculation are done in one central place.

Change-Id: Ib822cae0e9228e546b664fbac728a60d65c4bbc3
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit adc0920c48d812fb1b3e2b7bfc76217a7126a41e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qtimer.cpp

@@ -329,10 +329,7 @@ void QTimer::singleShotImpl(int msec, Qt::TimerType timerType,
 
 void QTimer::singleShot(int msec, const QObject *receiver, const char *member)
 {
-    // coarse timers are worst in their first firing
-    // so we prefer a high precision timer for something that happens only once
-    // unless the timeout is too big, in which case we go for coarse anyway
-    singleShot(msec, msec >= 2000 ? Qt::CoarseTimer : Qt::PreciseTimer, receiver, member);
+    singleShot(msec, defaultTypeFor(msec), receiver, member);
 }
 
 /*! \overload

src/corelib/kernel/qtimer.h

@@ -152,13 +152,20 @@ private:
     inline void killTimer(int){}
 
     static constexpr Qt::TimerType defaultTypeFor(int msecs) noexcept
-    { return msecs >= 2000 ? Qt::CoarseTimer : Qt::PreciseTimer; }
+    { return defaultTypeFor(std::chrono::milliseconds{msecs}); }
+
+    static constexpr Qt::TimerType defaultTypeFor(std::chrono::milliseconds interval) noexcept
+    {
+        // coarse timers are worst in their first firing
+        // so we prefer a high precision timer for something that happens only once
+        // unless the timeout is too big, in which case we go for coarse anyway
+        using namespace std::chrono_literals;
+        return interval >= 2s ? Qt::CoarseTimer : Qt::PreciseTimer;
+    }
+
     static void singleShotImpl(int msec, Qt::TimerType timerType,
                                const QObject *receiver, QtPrivate::QSlotObjectBase *slotObj);
 
-    static Qt::TimerType defaultTypeFor(std::chrono::milliseconds interval)
-    { return defaultTypeFor(int(interval.count())); }
-
     static void singleShotImpl(std::chrono::milliseconds interval, Qt::TimerType timerType,
                                const QObject *receiver, QtPrivate::QSlotObjectBase *slotObj)
     {