Remove all allows for diagnostic_outside_of_impl and untranslatable_diagnostic throughout the codebase

This PR was mostly made by search&replacing

src/lib.rs

@@ -1,6 +1,4 @@
 // tidy-alphabetical-start
-#![allow(rustc::diagnostic_outside_of_impl)]
-#![allow(rustc::untranslatable_diagnostic)]
 // Note: please avoid adding other feature gates where possible
 #![feature(rustc_private)]
 // Only used to define intrinsics in `compiler_builtins.rs`.