QStringLiteral: Remove const temporary that may prevent optimization

clang-tidy 10 was complaining about
http://clang.llvm.org/extra/clang-tidy/checks/performance-no-automatic-move.html

Change-Id: Iea5276e401a10f3ead8599e135dec1f0fa63a0dc
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qstringliteral.h

@@ -69,8 +69,7 @@ Q_STATIC_ASSERT_X(sizeof(qunicodechar) == 2,
             Q_STATIC_STRING_DATA_HEADER_INITIALIZER(Size), \
             QT_UNICODE_LITERAL(str) }; \
         QStringDataPtr holder = { qstring_literal.data_ptr() }; \
-        const QString qstring_literal_temp(holder); \
-        return qstring_literal_temp; \
+        return QString(holder); \
     }()) \
     /**/