QDateTimeParser: enable RVO in format()

Change-Id: I3d9f84d03519b2b8fb37c7d2e773e68bc4b9a1d3
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/tools/qdatetimeparser.cpp

@@ -1555,10 +1555,7 @@ QString QDateTimeParser::SectionNode::format() const
         qWarning("QDateTimeParser::sectionFormat Internal error 2");
         return QString();
     }
-
-    QString str;
-    str.fill(fillChar, count);
-    return str;
+    return QString(count, fillChar);
 }