Auto merge of #2237 - RalfJung:ice, r=oli-obk

add ICE error level

Fixes https://github.com/rust-lang/miri/issues/2235

But I am not sure if anything else should be adjusted when adding a new error level...

ui_test/src/rustc_stderr.rs

@@ -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 @@ fn from_str(s: &str) -> Result<Self, Self::Err> {
             "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}`")),
         }
     }