QAuthenticator: use QSB more

avoid temp allocations

Change-Id: I8ca4d4b748f79d1c7a4455a5105998c58c91c7a4
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/kernel/qauthenticator.cpp

@@ -555,7 +555,7 @@ QByteArray QAuthenticatorPrivate::calculateResponse(QByteArrayView requestMethod
         break;
     case QAuthenticatorPrivate::Basic:
         methodString = "Basic";
-        response = user.toLatin1() + ':' + password.toLatin1();
+        response = (user + ':'_L1 + password).toLatin1();
         response = response.toBase64();
         phase = Done;
         break;