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.

src/lib.rs

@@ -3,6 +3,8 @@
 #![cfg_attr(doc, doc(rust_logo))]
 #![feature(rustc_private)]
 // Note: please avoid adding other feature gates where possible
+#![allow(rustc::diagnostic_outside_of_impl)]
+#![allow(rustc::untranslatable_diagnostic)]
 #![warn(rust_2018_idioms)]
 #![warn(unused_lifetimes)]
 #![warn(unreachable_pub)]