[EH] Fuzzer: Fix infinite loop with nested exnrefs

[kripken]

When we need an exnref value we may create a try-catch with a throw (so
when we catch it, we get an exnref). But if the exception tag contains an
exnref, then we will try to create another exnref in the throw, leading to
recursion (possibly infinite). To avoid this, just create a trivial tag with no
values when we just want a trivial value.

[aheejin]

LGTM. By the way I think we had a restriction that that when GC is not enabled, a tag type cannot contain exnref because it can for cycles (and when there's no GC we have to rely on refcounting)... But I don't think we ended up actually writing that in the spec because we don't have "EH spec without GC" version. Not sure whether that's worth implementing in the fuzzing purpose though.

[kripken]

Oh, interesting about cycles. I guess disallowing exnref in tags in the fuzzer could have also avoided this, at least for non-GC... but the PR should handle the GC case too, so that's probably enough.