Rollup merge of rust-lang#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```

src/errors.rs

@@ -122,7 +122,7 @@ impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for TargetFeatureDisableOrEnabl
             diag.span(span);
         };
         if let Some(missing_features) = self.missing_features {
-            diag.subdiagnostic(missing_features);
+            diag.subdiagnostic(dcx, missing_features);
         }
         diag.arg("features", self.features.join(", "));
         diag