Use trailing zeros for `cir.const_struct`

[bcardosolopes]

          Shall we extend the work in https://github.com/llvm/clangir/pull/393 for trailing zeros in const structs? Not a blocker, but I'd like to have that consistency between arrays and structs (you could even use an interface if necessary?)

Originally posted by @bcardosolopes in #412 (comment)