Allow internal_features lint in doc tests

Rustdoc forwards -Zcrate-attr=feature(rustdoc_internals) to doc tests,
but deny(warnings) overrides the -Ainternal_features.

Author: bjorn3

compiler/rustc_public/src/lib.rs

@@ -7,7 +7,7 @@
 //! This API is still completely unstable and subject to change.
 
 #![allow(rustc::usage_of_ty_tykind)]
-#![doc(test(attr(allow(unused_variables), deny(warnings))))]
+#![doc(test(attr(allow(unused_variables), deny(warnings), allow(internal_features))))]
 #![feature(sized_hierarchy)]
 //!
 //! This crate shall contain all type definitions and APIs that we expect third-party tools to invoke to