Add "async" when using Generate Method on an await expression

[DustinCampbell]

using System;
using System.Threading.Tasks;

class C
{
    async Task M1(int id)
    {
        await M2(id);$$
    }
}

Press Ctrl+ at the caret and apply "Generate method 'C.M2'".

Expected Result:

private async Task M2(int id)
{
    throw new NotImplementedException();
}

Actual Result:

private Task M2(int id)
{
    throw new NotImplementedException();
}