Nightly diagnostic panic

[Juici]

The macro is panicking when emitting diagnostics on nightly.

error: proc-macro derive panicked
 --> tests/ui/missing_discriminant.rs:4:23
  |
4 | #[derive(Clone, Copy, IntEnum)]
  |                       ^^^^^^^
  |
  = help: message: assertion failed: child.level.can_be_top_or_sub().1

This is caused by a compiler change here https://github.com/rust-lang/rust/blob/59e0bc2de7134a2d88e9c14db32884e631e90373/compiler/rustc_errors/src/emitter.rs#L2119.

The only levels that can be used for sub-diagnostics emitted by proc-macros are now Warning, Note, and Help.