Rollup merge of #133702 - RalfJung:single-variant, r=oli-obk

Variants::Single: do not use invalid VariantIdx for uninhabited enums

~~Stacked on top of rust-lang/rust#133681, only the last commit is new.~~

Currently, `Variants::Single` for an empty enum contains a `VariantIdx` of 0; looking that up in the enum variant list will ICE. That's quite confusing. So let's fix that by adding a new `Variants::Empty` case for types that have 0 variants.

try-job: i686-msvc