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_borrowck/src/diagnostics/conflict_errors.rs

@@ -1,5 +1,8 @@
 // ignore-tidy-filelength
 
+#![allow(rustc::diagnostic_outside_of_impl)]
+#![allow(rustc::untranslatable_diagnostic)]
+
 use either::Either;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxIndexSet;