[BUG] DoNotOptimize(T &&) should not be deprecated

[davidstone]

Describe the bug

In #1493, the T const & overload of DoNotOptimize was deprecated because the compiler is still able to make assumptions about the value of the variable. However, it is unclear whether this concern would apply to an rvalue reference overload.

To reproduce

int f() {
    return 5;
}
void g() {
    benchmark::DoNotOptimize(f());
}

Expected behavior

No warning is emitted.

[dmah42]

@ckennelly is the expert. i defer to them.

[horenmar]

Bumping this because I share @davidstone's opinion and also because I have to deal with about 50 warnings caused by this change.