various: translation resources from cg backend

Extend `CodegenBackend` trait with a function returning the translation
resources from the codegen backend, which can be added to the complete
list of resources provided to the emitter.

Signed-off-by: David Wood <david.wood@huawei.com>

src/librustdoc/core.rs

@@ -115,8 +115,10 @@ pub(crate) fn new_handler(
     diagnostic_width: Option<usize>,
     unstable_opts: &UnstableOptions,
 ) -> rustc_errors::Handler {
-    let fallback_bundle =
-        rustc_errors::fallback_fluent_bundle(rustc_driver::DEFAULT_LOCALE_RESOURCES, false);
+    let fallback_bundle = rustc_errors::fallback_fluent_bundle(
+        rustc_driver::DEFAULT_LOCALE_RESOURCES.to_vec(),
+        false,
+    );
     let emitter: Box<dyn Emitter + sync::Send> = match error_format {
         ErrorOutputType::HumanReadable(kind) => {
             let (short, color_config) = kind.unzip();