Add a cir.const_vec, simlar to cir.const_array and cir.const_struct

[bcardosolopes]

          Random comment: If this was a constant vector (which isn't the case), this reminds me of something we discussed in previous patches it would be good to have `#cir.const_vec` of sorts, and similarly to `ConstArrayAttr`, we could have a trailing zero indicator. Adding a comment here so I can populate an issue.

Originally posted by @bcardosolopes in https://github.com/llvm/clangir/pull/490#discussion_r1513384939