Allow allow enums like `Result<T, E>` to be used across FFI if the
T/E can be niche optimized and the non-niche-optimized type is FFI safe.

Fields are disallowed so checking the top attribute is enough.

Tests both `T` and `E` for niche variant optimization lints

Couldn't find documentation supporting that single-variant
`#[repr(Rust)]` enums with RHS assigned work as expected with this
change.

```rust
enum Variants {
  A = 17,
} // Would this be zero sized optimized guaranteed?
```

Rollup of 6 pull requests

Successful merges:

 - rust-lang#124146 (Triagebot: Rename `macos` ping group to `apple`)
 - rust-lang#124742 (Add `rustfmt` cfg to well known cfgs list)
 - rust-lang#124745 (Implement lldb formattter for "clang encoded" enums (LLDB 18.1+) (v2))
 - rust-lang#124747 (Support Result<T, E> across FFI when niche optimization can be used (v2))
 - rust-lang#124765 ([rustdoc] Fix bad color for setting cog in ayu theme)
 - rust-lang#124768 ([resubmission] Meta: Enable the brand new triagebot transfer command)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 5 pull requests

Successful merges:

 - rust-lang#124742 (Add `rustfmt` cfg to well known cfgs list)
 - rust-lang#124747 (Support Result<T, E> across FFI when niche optimization can be used (v2))
 - rust-lang#124753 (Migrate `run-make/rustdoc-error-lines` to new `rmake.rs`)
 - rust-lang#124765 ([rustdoc] Fix bad color for setting cog in ayu theme)
 - rust-lang#124768 ([resubmission] Meta: Enable the brand new triagebot transfer command)

r? `@ghost`
`@rustbot` modify labels: rollup