Prefer QDate::startOfDay() over QDateTime(const QDate &)

The latter can be invalid if midnight is skipped by a spring-forward.

Change-Id: Ibf98d165557229f19622774ebf9a27bb0911c7a7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qdatetime.cpp

@@ -5214,7 +5214,7 @@ QDateTime QDateTime::fromString(const QString &string, Qt::DateFormat format)
         if (!date.isValid())
             return QDateTime();
         if (size == 10)
-            return QDateTime(date);
+            return date.startOfDay();
 
         Qt::TimeSpec spec = Qt::LocalTime;
         QStringRef isoString(&string);