Add missing DiagnosticBuilder::eager_diagnostic method.

This lets us avoid the use of `DiagnosticBuilder::into_diagnostic` in
miri, when then means that `DiagnosticBuilder::into_diagnostic` can
become private, being now only used by `stash` and `buffer`.

Author: nnethercote

src/diagnostics.rs

@@ -499,22 +499,20 @@ pub fn report_msg<'tcx>(
         err.note(if extra_span { "BACKTRACE (of the first span):" } else { "BACKTRACE:" });
     }
 
-    let (mut err, handler) = err.into_diagnostic().unwrap();
-
     // Add backtrace
     for (idx, frame_info) in stacktrace.iter().enumerate() {
         let is_local = machine.is_local(frame_info);
         // No span for non-local frames and the first frame (which is the error site).
         if is_local && idx > 0 {
-            err.eager_subdiagnostic(handler, frame_info.as_note(machine.tcx));
+            err.eager_subdiagnostic(err.dcx, frame_info.as_note(machine.tcx));
         } else {
             let sm = sess.source_map();
             let span = sm.span_to_embeddable_string(frame_info.span);
             err.note(format!("{frame_info} at {span}"));
         }
     }
 
-    handler.emit_diagnostic(err);
+    err.emit();
 }
 
 impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {