chore: legitimize projections on tagged values

[hargoniX]

This PR allows projections on tagged values in the IR type system.

While executing this branch of code should indeed never happen in practice, enforcing this through
the type system would require the compiler to always optimize code to the point where this is not
possible. For example in the code:

cases x with
| none => ....
| some =>
    let val : obj := proj[0] x
    ...

static analysis might learn that x is always none and transform this to:

let x : tagged := none
cases x with
| none => ....
| some =>
    let val : obj := proj[0] x
    ...

Which would be type incorrect if projections on tagged were illegitimate. However, we don't want
to force static analysis to always simplify code far enough on its own to enforce this invariant.