QTranslator: optimize string handling in find_transformation

- Don't repeatedly truncate a (shared) QString, truncate a QStringRef
  instead, preventing a detach.

Change-Id: I1a9cf7fc5bc9ea06279f7e2548f2bd144b8780a3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/kernel/qtranslator.cpp

@@ -668,7 +668,8 @@ static QString find_translation(const QLocale & locale,
     }
 
     // start guessing
-    for (QString localeName : qAsConst(fuzzyLocales)) {
+    for (const QString &fuzzyLocale : qAsConst(fuzzyLocales)) {
+        QStringRef localeName(&fuzzyLocale);
         for (;;) {
             int rightmost = localeName.lastIndexOf(QLatin1Char('_'));
             // no truncations? fail