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_index/src/lib.rs

@@ -1,5 +1,3 @@
-#![deny(rustc::untranslatable_diagnostic)]
-#![deny(rustc::diagnostic_outside_of_impl)]
 #![cfg_attr(
     feature = "nightly",
     feature(extend_one, min_specialization, new_uninit, step_trait, test)