Handle new caller_location intrinsic the right way

[adpaco-aws]

Rebase from #346 found a new caller_location intrinsic in the process. In order to unblock, we codegen this intrinsic as:

            "caller_location" => {
                codegen_unimplemented_intrinsic!("https://github.com/model-checking/rmc/issues/374")
            }

But we can probably codegen it the same way Rust does.

[celinval]

This is a nice explanation on what this does: https://hegdenu.net/posts/track-caller/

[adpaco-aws]

Thanks for the update, @celinval !

If adding support for caller_location would allow us to avoid panic messages such as thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', that'd be a major improvement in how we report such failures.

One thing that I don't see mentioned there is the relation to panic strategies. This isn't mentioned in the RFC neither, so I guess it's independent.

[celinval]

I was wondering if it would help with: #1740

[celinval]

For the current Rust implementation, the compiler will add an extra argument to functions that contain #[track_caller]. Unfortunately, this operation is done in the codegen stage, instead of a MIR pass. :(

My guess is that MIR passes can't really change the function signature, so they push it down to the codegen stage. I'm tempted to just encode this as a global variable for now.

@adpaco-aws I don't think CBMC really supports formatting error messages, so I'm not sure implementing this intrinsic will make much of a difference on how we report failures.

In order to fix that, we would have to either inline these functions or generate a different copy per location they are invoked from.

[celinval]

Implementation notes for myself:

1. To build a constant that represents a location, use query const_caller_location [1].
2. To get the type of the caller location, use function: pub fn caller_location_ty(self) -> Ty<'tcx> [2]
3. We can check if a function requires caller location using requires_caller_location [3]
4. If we are passing the caller location as an argument, we will need to create a ReifyShim for cases that require dynamic dispatch.