rustc_metadata: Encode doc(hidden) flag to metadata

To retrieve these flags rustdoc currently has to mass decode full attributes for items in the whole crate tree, so it's better to pre-compute it in advance.

This is especially for short-term performance of https://github.com/rust-lang/rust/pull/107054 because resolver cannot use memoization of query results yet.

compiler/rustc_middle/src/query/mod.rs

@@ -1157,6 +1157,7 @@ rustc_queries! {
     /// Determines whether an item is annotated with `doc(hidden)`.
     query is_doc_hidden(def_id: DefId) -> bool {
         desc { |tcx| "checking whether `{}` is `doc(hidden)`", tcx.def_path_str(def_id) }
+        separate_provide_extern
     }
 
     /// Determines whether an item is annotated with `doc(notable_trait)`.