Async keyword not highlighted on local functions

Repro:
```c#
using System.Threading.Tasks;

class C
{
    async Task M()
    {
        async Task F()
        {
            await Task.Yield();
        }

        await F();
    }
}
```
Click on first `async` - both `async` and corresponding `await` is highlighted.
Click on second `async` - nothing is highlighted.