Print diagnostics at runtime for Invalid

[mshinwell]

This makes it much easier to fix occurrences of Invalid at runtime. For example:

% cat inv.ml
type t = {
  mutable x : float;
}

let foo () =
  let t = 1, 2 in
  Array.unsafe_get ((Obj.magic t) : int array) 0

let (_ : int) = foo ()
% ./inv
[ocaml] [flambda2] Invalid code:
(Defining_expr_of_let (bound_pattern Parrayrefu/22)
 (defining_expr
  (((Array_load Immediates Mutable) Inv.camlInv__const_block9 0)
   inv.ml:7,2--48)))

This might have arisen from a wrong use of [Obj.magic].
Consider using [Sys.opaque_identity].
zsh: abort      ./inv

I've removed the flags for the distinction between "halt and catch fire" and "treat as unreachable". I think we should just choose the right semantics. There is one place that makes a distinction between these two settings at the moment -- I have maintained this as it was for "halt and catch fire", lest the alternative causes problems. I have left a CR to fix this later however.

I was wondering about just using exit but in the end decided abort is probably better: it's possible that the exact address of failure might still be useful, in case of ambiguity in the printed diagnostic.