QLocale(Win): fix pessimizing use of QStringBuilder

Instead of

   result += QString(string builder expression);

forcing a QString creation incl. memory allocation, do

   result += string builder expression;

using the overloaded QString += QStringBuilder operator.

Change-Id: I23023c76620fa6bb7bf9f2786c22f6a2ec0d87c2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/corelib/tools/qlocale_win.cpp

@@ -691,7 +691,7 @@ QString QSystemLocalePrivate::winToQtFormat(const QString &sys_fmt)
             if (text == QLatin1String("'"))
                 result += QLatin1String("''");
             else
-                result += QString(QLatin1Char('\'') + text + QLatin1Char('\''));
+                result += QLatin1Char('\'') + text + QLatin1Char('\'');
             continue;
         }