Implement RFC 3631: add rustdoc doc_cfg features

[GuillaumeGomez]

Implementation of rust-lang/rfcs#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) impls 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 cfgs. 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

[petrochenkov]

What remains to be done:

I'd also want to block the stabilization on landing #138844 to avoid a stable rustdoc feature relying on externally observable hacks in rustc.
The crater run in #138844 returned mostly clean, so I expect it to land soon.

[GuillaumeGomez]

Noted! And that will be a nice improvement, thanks!

Just one thing left for the cfg expansion missing: #[cfg_attr(blabla, derive(Debug))]. In this case, the cfg is not kept in the generated derive items. It's been in my TODO list for a long time now. ^^'

[petrochenkov]

Just one thing left for the cfg expansion missing: #[cfg_attr(blabla, derive(Debug))]

Do you mean like in #138515? :)

[GuillaumeGomez]

You're my hero! Gonna need to handle this new attribute then. :)

[bors]

☔ The latest upstream changes (presumably #138923) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #138927) made this pull request unmergeable. Please resolve the merge conflicts.