Also call MustTreatTypeAsGenericInstType for next node of TypeSig

e.g.
Import method 'Task.ContinueWith'
``` cs
public class Task<TResult> {
	public Task ContinueWith(Action<Task<TResult>> continuationAction);
}
```
After importing, 'Action<Task<TResult>>' will be 'Action<Task`1>' (generic instantiation becomes generic definition).