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

compiler/rustc_lexer/src/lib.rs

@@ -18,6 +18,8 @@
 //! lexeme types.
 //!
 //! [`rustc_parse::lexer`]: ../rustc_parse/lexer/index.html
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 // We want to be able to build this crate with a stable compiler, so no
 // `#![feature]` attributes should be added.