[Custom Descriptors] Handle a null descriptor in Heap2Local

[kripken]

The type must change to unreachable in this case: the ref.get_desc
will not return a descriptor of the proper type, and we trap.

Found by #7913

[tlively]

I don't get why this is necessary. If ref.get_desc would return a null, then the original allocation that installed the null descriptor would already have trapped, so the ref.get_desc will never be reached.

[kripken]

But the type is not right. We cannot return ref.null for a place that expects (ref (exact $desc)).

[kripken]

This traps, as you say. But the ref.null below is what determines the type of the entire expression, and it is wrong without the unreachable this PR appends.

[tlively]

Oh, I see, thanks. So another solution would be to add a ref.as_non_null, but adding unreachable is no more complex and gives better results.

[kripken]

Could ref.as_non_null work? It just affects nullability, but the problem is we can't return an exact descriptor since we don't have one.

[tlively]

But none <: (exact $desc), so the only problem here is the nullability.

[kripken]

Oh right, I forgot exact types do have one subtype...