Fix false negative in DisposedBeforeAsyncRunAnalyzer

[dawedawe]

fixes #75

[nojaf]

Maybe make a note that this whole thing isn't tail-recursive? Or that is at least my educated guess.

Should we include SynExpr.MatchBang here?

[dawedawe]

From what I see, that's already covered because to use match! we would need to be in an async already like:

let blah = new DisposableThing ()
use blah = blah
async {
    match! async {return 1} with
    | _ -> async { return "hi" }
}

[nojaf]

Yeah, but in theory we could have something like:

let meh = async {
  let blah = new DisposableThing ()
  use blah = blah
  return! async {
      match! async {return 1} with
      | _ -> async { return "hi" }
  }
}

Might not be relevant for now.

[nojaf]

Ship it!