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_ty_utils/src/representability.rs

@@ -1,5 +1,3 @@
-#![allow(rustc::untranslatable_diagnostic, rustc::diagnostic_outside_of_impl)]
-
 use rustc_hir::def::DefKind;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::query::Providers;