returning __rvalue should move-construct the NRVO value

[WalterBright]

T returnRval(ref T arg)
{
//    return __rvalue(arg); // doesn't work; should move-construct the NRVO value

    // this does work as expected:
    T t = __rvalue(arg); // move-construct a temp
    return t;            // return temp via NRVO
}

[WalterBright]

From @TurkeyMan #17050