Disallow descriptor "L" subtyping (#9011)

The custom descriptors proposal previously allowed subtyping that looked
like this:

```
A
^
B -> B.desc
```

It was possible for a type to have a descriptor while its supertype did
not.

However, we've decided to tighten the validation rules to disallow this
pattern. Now `A`` will also need a descriptor, `A.desc`, and `A.desc`
will need to be the declared supertype of `B.desc` for this to be valid.

See https://github.com/WebAssembly/custom-descriptors/issues/109.