Fix annotated join with non-concrete eltype iters (#54919)
As raised in
<https://github.com/JuliaLang/StyledStrings.jl/issues/57#issuecomment-2097457142>,
when the eltype of an iterator is non-concrete, `_isannotated` will
return false. To properly check such cases, we need to see if any of the
elements of the iterator are annotated.
This is a bit of an interesting case, since:
- Most of the time it shouldn't be hit, we can reasonably expect most
iterables to infer as producing concrete types
- The eltype of the iterator is (generally) known at compile-time, and
so in any case other than the ambiguous non-concrete one, this check
remains able to be done at compile-time.
With this change, join always preserves annotations. The compromise made
is that iterators with non-concrete eltypes can result in join inferring
a union return type (i.e. type instability with non-concrete iterators),
but that doesn't seem too bad to me (I don't see how we could be
completely type stable without concrete types here).
