Fix https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1442700

From the feedback:

Start with this code:

void Foo()
{
    Task.Run(async () => $$
}

Then type await in the Task.Run lambda (include the space after await).

Expected:

No change to Foo’s signature.

Actual:

Now it’s been updated to

async void Foo()

This behavior is generating errors (adding async where it shouldn’t be) and doing so arbitrarily far from my current cursor, and it’s really distracting to have to track these down and delete.