[llvm] [ADT] Fix llvm::concat_iterator for `ValueT == common_base_class *` (PR #144744)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

I think the two pieces of feedback haven't been addressed yet:
1. I think we should test whether the values we iterate over are references to elements or not
2. For containers of the same pointer type as in `concat<T *>`, I think we should keep existing behavior and allow for modification

https://github.com/llvm/llvm-project/pull/144744