QByteArray benchmark: fix clang -Wmove-result

Take the argument by value instead of rvalue ref. In C++17, this is as
efficient, and enables the

   return big;

to use the QByteArray move constructor, avoiding the need for a manual
std::move(), which always looks suspicious on a return statement,
because more often than not, it's a pessimization that breaks NRVO.

This code doesn't seem to exist in Qt 6.2, so only

Pick-to: 6.3
Change-Id: I8bf678102f5df1870cfc61090d12f327478d74d1
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

tests/benchmarks/corelib/text/qbytearray/tst_bench_qbytearray.cpp

@@ -91,7 +91,7 @@ void tst_QByteArray::append()
     }
 }
 
-static QByteArray decNext(QByteArray &&big)
+static QByteArray decNext(QByteArray big)
 {
     // Increments a decimal digit-string (ignoring sign, so decrements if
     // negative); only intended for taking a boundary value just out of range,