Enum abstract values can be typed as the underlying type

It's currently possible to type enum abstract values as the underlying type if there's a `from`, which seems like a strange misuse that can unfortunately be found in some codebases...

```haxe
function main() {
switch Test.Foo {
case "":
}
}

enum abstract Test(String) from String {
var Foo:String;
}
```

Nothing is ever typed as `Test` here.