Invert diagnostic lints.

That is, change `diagnostic_outside_of_impl` and
`untranslatable_diagnostic` from `allow` to `deny`, because more than
half of the compiler has be converted to use translated diagnostics.

This commit removes more `deny` attributes than it adds `allow`
attributes, which proves that this change is warranted.

compiler/rustc_errors/src/lib.rs

@@ -5,6 +5,8 @@
 // tidy-alphabetical-start
 #![allow(incomplete_features)]
 #![allow(internal_features)]
+#![allow(rustc::diagnostic_outside_of_impl)]
+#![allow(rustc::untranslatable_diagnostic)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(array_windows)]