AnnotatedIOBuffer: prevent despecialization of annotations list (#53155)
In f117a500ca93 code modifying annotations via list comprehensions was
introduced. However, when (1) there are annotations present and (2) all
annotations are filtered out, this comprehension is inferred to a wider
type incompatible with the AnnotatedString constructor. This can be
fixed by changing the first element of the tuple to directly use the
UnitRange{Int} constructor, which stops it being inferred as an Any.