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

@@ -27,8 +27,6 @@
 #![recursion_limit="256"]
 #![warn(rust_2018_idioms)]
 #![warn(unused_lifetimes)]
-#![deny(rustc::untranslatable_diagnostic)]
-#![deny(rustc::diagnostic_outside_of_impl)]
 
 extern crate rustc_apfloat;
 extern crate rustc_ast;