Unsound: TR accepts non-productive type aliases/definitions

A repro:

```racket
#lang typed/racket/base
(define-type A B)
(define-type B (Union A Symbol))

(: a1 A)
(define a1 'a)

(: a2 A)
(define a2 123)

(: a3 Symbol)
(define a3 a2)
```

```
> a3
- : Symbol
123
```