Infrastructure: Add mechanism to validate SMIR

[celinval]

We need a better mechanism to validate StableMIR translation. Maybe a mini-interpreter or at least create a debug option that we can inspect.

[oli-obk]

As a minimal first step we could create a dummy "codegen" that doesn't even monomorphize, but just tries to generate the MIR of all public items and all items needed by them.

Edit: ah you already said that in the linked issue

[celinval]

@rustbot claim

[celinval]

@rustbot claim

[celinval]

We added a regression suite and and dummy driver that inspect and test SMIR in #25