De-weirdify fatally_break_rust.

The easter egg ICE on `break rust` is weird: it's the one ICE in the
entire compiler that doesn't immediately abort, which makes it
annoyingly inconsistent.

This commit changes it to abort. As part of this, the extra notes are
now appended onto the bug dignostic, rather than being printed as
individual note diagnostics, which changes the output format a bit.
These changes don't interferes with the joke, but they do help with my
ongoing cleanups to error handling.

compiler/rustc_errors/src/lib.rs

@@ -968,6 +968,19 @@ impl DiagCtxt {
         DiagnosticBuilder::new(self, Level::Bug, msg)
     }
 
+    /// Construct a builder at the `Bug` level at the given `span` with the `msg`.
+    #[rustc_lint_diagnostics]
+    #[track_caller]
+    pub fn struct_span_bug(
+        &self,
+        span: impl Into<MultiSpan>,
+        msg: impl Into<DiagnosticMessage>,
+    ) -> DiagnosticBuilder<'_, BugAbort> {
+        let mut result = self.struct_bug(msg);
+        result.set_span(span);
+        result
+    }
+
     #[rustc_lint_diagnostics]
     #[track_caller]
     pub fn span_fatal(&self, span: impl Into<MultiSpan>, msg: impl Into<DiagnosticMessage>) -> ! {