message: add ICE error level
diff: @@ -16,6 +16,7 @@ struct RustcMessage {
  #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
  pub(crate) enum Level {
  + Ice = 5,
  Error = 4,
  Warn = 3,
  Help = 2,
  @@ -52,6 +53,7 @@ impl std::str::FromStr for Level {
  "HELP" | "help" => Ok(Self::Help),
  "NOTE" | "note" => Ok(Self::Note),
  "failure-note" => Ok(Self::FailureNote),
  + "error: internal compiler error" => Ok(Self::Ice),
  _ => Err(format!("unknown level `{s}`")),
  }
  }
  