pretty/mir: const value enums with no variants
Fixes#72181.
This PR modifies the pretty printer and const eval in the MIR so that `destructure_const` (used in `pretty_print_const_value`) can handle enums with no variants (or types containing enums with no variants).
I'm not convinced that this is the correct approach, folks more familiar with `destructure_const` would be able to say - happy to adjust the PR. Looking through `destructure_const` and the functions that it invokes, it didn't seem like it was written to handle zero-variant-enums - I assume that case is handled earlier in some way so `destructure_const` doesn't need to under normal circumstances. It didn't seem like it would be straightforward to make `destructure_const` handle this case in a first-class-feeling way (e.g. adding a `Variants::None` variant), so this PR makes some minimal changes to avoid ICEs.
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