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

compiler/rustc_ast/src/lib.rs

@@ -19,6 +19,8 @@
 #![feature(slice_internals)]
 #![feature(stmt_expr_attributes)]
 #![recursion_limit = "256"]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 
 #[macro_use]
 extern crate rustc_macros;