Rollup merge of #60529 - davidtwco:rfc-2008-uninhabited, r=petrochenkov

RFC 2008: Uninhabitedness fixes for enum variants and tests

Part of #44109.

At the request of @Centril, this PR adds tests asserting that uninhabited non-exhaustive types are considered inhabited in extern crates. In adding these tests, I fixed an oversight in the implementation of RFC 2008 on enum variants that resulted in non-exhaustive enum variants being considered uninhabited in extern crates.

Before this PR, these lines would error:

```rust
// extern crate
pub enum UninhabitedVariants {
    #[non_exhaustive] Tuple(!),
    #[non_exhaustive] Struct { x: ! }
}

pub enum PartiallyInhabitedVariants {
    Tuple(u8),
    #[non_exhaustive] Struct { x: ! }
}

// current crate
match uninhabited_variant() /* fn() -> Option<UninhabitedVariants> */ {
    Some(_x) => (), //~ ERROR unreachable pattern
    None => (),
}

while let PartiallyInhabitedVariants::Struct { x, .. } = partially_inhabited_variant() /* fn() -> PartiallyInhabitedVariants */ {
    //~^ ERROR unreachable pattern
}
```

cc @Centril
r? @petrochenkov

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