errors: generate typed identifiers in each crate

Instead of loading the Fluent resources for every crate in
`rustc_error_messages`, each crate generates typed identifiers for its
own diagnostics and creates a static which are pulled together in the
`rustc_driver` crate and provided to the diagnostic emitter.

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

Author: davidtwco

src/parse/session.rs

@@ -124,7 +124,7 @@ fn default_handler(
         silent_emitter()
     } else {
         let fallback_bundle =
-            rustc_errors::fallback_fluent_bundle(rustc_errors::DEFAULT_LOCALE_RESOURCES, false);
+            rustc_errors::fallback_fluent_bundle(rustc_driver::DEFAULT_LOCALE_RESOURCES, false);
         Box::new(EmitterWriter::stderr(
             color_cfg,
             Some(source_map.clone()),