Type of Erased types #2914

Right now any type decorated with Erase becomes Any in Fable AST. This makes sense because originally Erase was thought for erased unions representing Typescript unions. However now there are cases where we want to erase the declaration but not necessarily the type.

Another use case is when using a union just to wrap a primitive, then it should probably become the type of the primitive (although I'm not sure if Fable users frequently do this).