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).

Signed-off-by: David Wood <david@davidtw.co>

src/errors.rs

@@ -122,7 +122,7 @@ fn into_diagnostic(self, dcx: &'_ DiagCtxt, level: Level) -> DiagnosticBuilder<'
             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