async function containing locally abstract type is not considered an async context

### Repro

```js
let f = async (type a, ()) => {
  await Js.Promise.resolve(())
}
```

causes the error:

```none
Await on expression not in an async context
```

### Workaround

Make the function explicitly curried and move the async specifier after the locally abstract type.

```js
let f = (type a) => async () => {
  await Js.Promise.resolve(())
}
```