Confusing error message when missing parentheses in let-bound pattern match and tree structure

Consider the following:

```fsharp
type T = Node of T *T
module T =
let f t =
let Node(l, r) = t
l, r |> ignore
```

**Expected:**

This either:

* Works just fine
* Gives an error at the `let Node(l, r)` site

**Actual:**

This gives an error on `l` and `r` saying they're undefined:

The fix is to wrap the let-bound pattern match in parentheses:

```fsharp
type T = Node of T *T
module T =
let f t =
let (Node(l, r)) = t
l, r |> ignore
```