QLocale: replace use of append(QString(n, c)) with resize(size() + n, c)

Avoids a temporary QString creation.

Change-Id: Ibc87a3f1bf2979780e4a2cce7e1757f97c39a9be
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qlocale.cpp

@@ -3565,7 +3565,7 @@ QString QCalendarBackend::dateTimeToString(QStringView format, const QDateTime &
             }
         }
         if (!used)
-            result.append(QString(repeat, c));
+            result.resize(result.size() + repeat, c);
         i += repeat;
     }