hidden variable names leak into normal namespace

```fsharp
let add _ _ = _arg1 + _arg2

[]
let main _ =
printfn "%d" (add 1 2)
0
```

The above snippet compiles, runs and works (as expected). I imagine this is an oversight, buggy behavior that was not intended to happen. Specifically, being able to alias the arguments denoted by the ignore pattern with `_arg[n]` syntax.

On the plus side, tooling (vanilla VS2017) does not offer Intellisense or anything of the sort in this situation.