Auto merge of #138907 - GuillaumeGomez:rfc-3631, r=fmease,camelid,Manishearth,lolbinarycat

Implement RFC 3631: add rustdoc doc_cfg features

Implementation of https://github.com/rust-lang/rfcs/pull/3631.

This implementation actually resulted in a lot of simplifications:
 * All `cfg` computation is now done in one place: `propagate_doc_cfg.rs`. Because (trait) `impl`s are not retrieved at the same time as the other items, we cannot perform this computation in the clean process, it needs to be after.
 * Because there is `cfg` inheritance, we can keep track of them in one place (in `propagate_doc_cfg.rs`), meaning we don't need to copy an item's attributes to its children anymore. Only exception: impl items. For them we clone only `cfg` attributes.
 * `propagate_doc_cfg.rs` is also now much simpler, much less need to keep track of parents, since everything we need is handled by the new `CfgInfo` type.
 * I also suspect that `Cfg::simplify_with` could either be removed or at least used directly into `propagate_doc_cfg.rs` when we compute `cfg`s. Considering how big the PR already is, I'll do it in a follow-up.

I didn't remove the `doc_cfg*` features in this PR because some dependencies used in `rustc` (like `stdarch`) are using it, so we need to have a nightly released with this PR before I can switch to the new feature.

r? ghost