_desugarTypedef accepts a _RawType and returns its
underlying type if it's a typedef. However, it doesn't
account for whether the _RawType is nullable or not.
In order to accurately desugar a use of a typedef, we
should union the _RawType's nullability with the underlying
type's nullability. For example, `SomeTypedef?` should
always be nullable after being desugared.