NoSimpleLetBody should not report errors for a variable that is referenced outside of the body

[jfmengels]

fn value =
  let
    a =
      something

    something =
      1
  in
  something
--> should NOT transform to
fn value =
  let
    a =
      something
  in
  1

The above should not be reported. Even though a is necessarily unused (at least for the current implementation), it may still exist for a bit, which will cause the code not to compile anymore. It's likely that NoUnused.Variables is enabled and will fix the issue anyway, but we shouldn't rely on this.