Avoid recomputing DST-ness for time-zone when already known

When a QDateTime is constructed it determines whether daylight-saving
time is in effect and records it in its status field. So consult that
cached answer first, before asking the time-zone backend to work out
the answer the slow way.

Change-Id: I1d380d41f28a9b866216ac0f65425c30a8988cac
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/time/qdatetime.cpp

@@ -3613,6 +3613,10 @@ bool QDateTime::isDaylightTime() const
         break;
 #else
         Q_ASSERT(d->m_timeZone.isValid());
+        if (auto dst = extractDaylightStatus(getStatus(d));
+            dst != QDateTimePrivate::UnknownDaylightTime) {
+            return dst == QDateTimePrivate::DaylightTime;
+        }
         return d->m_timeZone.d->isDaylightTime(toMSecsSinceEpoch());
 #endif // timezone
     case Qt::LocalTime: {