Add an aliases for mutable types in CIR

[bcardosolopes]

          Unfortunately, mutable types do not support aliases yet, but there is a fix for this in review: https://github.com/llvm/llvm-project/pull/66663.

At some point, we will likely be able to use aliases on everything, so I suggest that, for now, we leave only the structs using full types and everything else using aliases.

Originally posted by @sitio-couto in https://github.com/llvm/clangir/pull/303#discussion_r1379492117