Layout for enums does not match MIR (and types) very well

[RalfJung]

Enum layout computation entirely ignores ZST uninhabited ("absent") variants, which can lead to the layout being quite different in structure from the user-visible enum:

• if there are more than 1 non-absent variants, Variants::Multiple is used.
• if there is exactly 1 non-absent variant, Variants::Single is used.
• if all variants are absent, the layout of ! is used, so the entire thing ends up looking like a union with 0 fields.

When actually asking for the layout of such an absent variant, it has to be made up "on the spot".

In terms of testing things like Miri (or anything else that needs to work with layout variants information), this leads to the "interesting" situation that we need an enum with at least 3 variants where 1 is "absent" to make sure that Variants::Multiple with absent variants is handled correctly.

This could be improved by generalizing Variants::Multiple to also support "discriminants" of "type" () and !, thus uniformly handling all enums.

Also see the second half of #69763 (comment), and discussion below #69763 (comment).

Cc @eddyb