Make `cir.array` types mutable to allow forward declarations

[bcardosolopes]

          @bcardosolopes maybe we should make `cir.array` types mutable too since they allow forward declarations. This would allow us to modify it in place instead of having to rewrite all its uses.

@Lancern no need to apply this review. It's here just for future reference.

Originally posted by @sitio-couto in https://github.com/llvm/clangir/pull/375#discussion_r1442165667