Uncaptured locals are not available in closures when debugging

Repro:

1. Open VisualFSharp.sln
2. Open SignatureHelp.fs
3. Put a breakoint on line 123(`let startOfArgs = ...`)
4. Launch a hive of VS where you can debug
5. Enter the following source code:

```fsharp
type C() =
static member M(x, y, z) = ()

C.M(1)
```

6. Place cursor after the `1` on the last line
7. Press the comma character (`,`) so that Signature Help is triggered

Note that there are some locals, and critically you can observe the value of `applicableSpan`:


Now try to get the source code that matches this span by:

1. Opening the Immediate Window
2. Entering the following:

`sourceText.GetSubText(applicableSpan)`

You will observe that `sourceText` doesn't exist. Indeed, it wasn't even in the locals window.


However, note that `sourceText` is a parameter to this method and it is clearly accessible.

We are losing debug info on every `do!` call in the method.

To test this, place a breakpoint on line 89 (`do! Option.guard (methods.Length > 0 ...`). When it is broken there, you'll see that `sourceText` is available: