When implementing interfaces, cancellable tasks can produce errors in the wrong places

**Repro steps**

Consider this code:

```fsharp
type ITest =
abstract member DoStuff: unit -> CancellableTask

type Test() =
interface ITest with
member _.DoStuff() =
cancellableTask {
let _test1 = "blah"

let! _test2 = CancellableTask.getCurrentCancellationToken()

return "blah"
}
```

**Expected behavior**

I would imagine the code producing an error either around the `cancellableTask` definition or around the last line of the CE.
Provide a description of the expected behavior.

**Actual behavior**

Actually the code produces an error at the first bang binding.


**Known workarounds**

None, just figuring out what's going on and fixing the error.

**Related information**

The error span seems reasonable when interfaces are not in the game: