Rollup merge of #128934 - Nadrieril:fix-empty-non-exhaustive, r=compiler-errors

Non-exhaustive structs may be empty

This is a follow-up to a discrepancy noticed in https://github.com/rust-lang/rust/pull/122792: today, the following struct is considered inhabited (non-empty) outside its defining crate:
```rust
#[non_exhaustive]
pub struct UninhabitedStruct {
    pub never: !,
    // other fields
}
```

`#[non_exhaustive]` on a struct should mean that adding fields to it isn't a breaking change. There is no way that adding fields to this struct could make it non-empty since the `never` field must stay and is inconstructible. I suspect this was implemented this way due to confusion with `#[non_exhaustive]` enums, which indeed should be considered non-empty outside their defining crate.

I propose that we consider such a struct uninhabited (empty), just like it would be without the `#[non_exhaustive]` annotation.

Code that doesn't pass today and will pass after this:
```rust
// In a different crate
fn empty_match_on_empty_struct<T>(x: UninhabitedStruct) -> T {
    match x {}
}
```

This is not a breaking change.

r? ``@compiler-errors``

compiler/rustc_middle/src/ty/inhabitedness/mod.rs

@@ -81,10 +81,6 @@ impl<'tcx> VariantDef {
         adt: ty::AdtDef<'_>,
     ) -> InhabitedPredicate<'tcx> {
         debug_assert!(!adt.is_union());
-        if self.is_field_list_non_exhaustive() && !self.def_id.is_local() {
-            // Non-exhaustive variants from other crates are always considered inhabited.
-            return InhabitedPredicate::True;
-        }
         InhabitedPredicate::all(
             tcx,
             self.fields.iter().map(|field| {