make `Base.ANSIIterator` have a concrete field (#56088)
Avoids the invalidation
```
backedges: 1: superseding sizeof(s::AbstractString) @ Base strings/basic.jl:177 with MethodInstance for sizeof(::AbstractString) (75 children)
```
shown in
https://github.com/JuliaLang/julia/issues/56080#issuecomment-2404765120.
Co-authored-by: KristofferC <kristoffer.carlsson@juliacomputing.com>
