Improved error message: `use y = fixed x` does not compile inside computation expressions


```fsharp
#nowarn "9"

open System

type TestCaseBuilder() =
member __.Using(disposable: IDisposable, f) =
try
f disposable
finally
match disposable with
| null -> ()
| disp -> disp.Dispose()

member __.Combine(f1, f2) = f2(); f1
member __.Zero() = ()
member __.Delay f = f
member __.Run f = f

let test = TestCaseBuilder()

[]
let main argv =
let x = [|1|]
use y = fixed x

test {
let x = [|1|]
let y = fixed x
use z = fixed x
()
}
|> ignore
0
```