`Rvalue::Repeat` should hold a `ty::Const`

[varkor]

fn test<const N: usize>() {
    let array = [0; N];
}

currently fails with error: array lengths can't depend on generic parameters, because we can't tell what concrete value N should take here. This is an artificial limitation: when producing a Rvalue::Repeat, we currently need to provide the length of the array, which is impossible at this stage.
cc #43408 (comment)

[eddyb]

cc @oli-obk @yodaldevoid