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 important for short-term performance of https://github.com/rust-lang/rust/pull/107054 because resolver cannot use memoization of query results yet.
5bef91c6e9