invalid_value misses some clearly bad enum initialization

[RalfJung]

The invalid_value lint has no issue with the following type being zero-initialized or left uninit:

enum OneFruitNonzero {
    Apple(!),
    Banana(NonZeroU32),
}

That seems like a case we should be able to handle.

I think for non-is_multi_variant enums, we probably can compute the variant that does exist, and then we can recursively check if that one is valid? We'd also need a smarter is_multi_variant that can handle some dataful variants (ensuring they are definitely inhabited).

Cc @5225225

[RalfJung]

Will be fixed by #102281

[Rageking8]

@rustbot label +C-bug +A-lint