1.99 beta crater regression: panic when formatting bitflags

[cyrgani]

log: https://crater-reports.s3.amazonaws.com/beta-1.99-5/1.99.0-beta.1/reg/extended-notify-0.1.3/log.txt
partial reduction:

use enumflags2::bitflags;

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[bitflags]
#[repr(u64)]
pub(crate) enum Interest {
    Established,
    Any,
    Access,
    AccessOpen,
    AccessClose,
    AccessRead,
    AccessOther,
    Create,
    CreateFile,
    CreateFolder,
    CreateOther,
    Modify,
    ModifyData,
    ModifyDataSize,
    ModifyDataContent,
    ModifyDataOther,
    ModifyMetadata,
    ModifyMetadataAccessTime,
    ModifyMetadataWriteTime,
    ModifyMetadataPermissions,
    ModifyMetadataOwnership,
    ModifyMetadataExtended,
    ModifyMetadataOther,
    ModifyRename,
    ModifyRenameTo,
    ModifyRenameFrom,
    ModifyRenameBoth,
    ModifyRenameOther,
    ModifyOther,
    Delete,
    DeleteFile,
    DeleteFolder,
    DeleteOther,
    Other,
}

fn main() {
    let flags = Interest::Established | Interest::Create;
    eprintln!("run step {flags:?}");
}

[cyrgani]

slightly smaller:

use enumflags2::bitflags;

#[derive(Debug, Clone, Copy)]
#[bitflags]
#[repr(u64)]
pub(crate) enum Interest {
    // we need at least 10 variants to trigger the faulty enum derive(Debug) optimization
    V0,
    V1,
    V2,
    V3,
    V4,
    V5,
    V6,
    V7,
    V8,
    V9,
}

fn main() {
    let flags = Interest::V0 | Interest::V4;
    eprintln!("run step {flags:?}");
}

The problem is that #155452 added a new optimization to derive(Debug) for fieldless enums. But applying #[bitflags] rewrites the enum and gives it discriminants, making the generated Debug code incorrect.
Essentially this is yet another instance of #148423.