Rollup merge of #121085 - davidtwco:always-eager-diagnostics, r=nnethercote

errors: only eagerly translate subdiagnostics

Subdiagnostics don't need to be lazily translated, they can always be eagerly translated. Eager translation is slightly more complex as we need to have a `DiagCtxt` available to perform the translation, which involves slightly more threading of that context.

This slight increase in complexity should enable later simplifications - like passing `DiagCtxt` into `AddToDiagnostic` and moving Fluent messages into the diagnostic structs rather than having them in separate files (working on that was what led to this change).

r? ```@nnethercote```

Author: matthiaskrgr

src/diagnostics.rs

@@ -505,7 +505,7 @@ pub fn report_msg<'tcx>(
         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(err.dcx, frame_info.as_note(machine.tcx));
+            err.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);