Add diagnostic translation lints to crates that don't emit them

compiler/rustc_error_messages/src/lib.rs

@@ -1,6 +1,8 @@
 #![feature(once_cell)]
 #![feature(rustc_attrs)]
 #![feature(type_alias_impl_trait)]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 
 use fluent_bundle::FluentResource;
 use fluent_syntax::parser::ParserError;