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>

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -131,7 +131,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                         // FIXME(effects) revisit this
                         if !tcx.is_const_trait_impl_raw(data.impl_def_id) {
                             let span = tcx.def_span(data.impl_def_id);
-                            err.subdiagnostic(errors::NonConstImplNote { span });
+                            err.subdiagnostic(tcx.dcx(), errors::NonConstImplNote { span });
                         }
                     }
                 }