Remaining issues after the `#[doc]` attribute port

From https://github.com/rust-lang/rust/pull/149645

• Some part of cfg_old.rs are likely unused now, so they should be removed. https://github.com/rust-lang/rust/pull/149903
• Not all error/lints are emitted at the same time anymore, making them kinda less useful considering that you need to run and fix rustc/rustdoc multiple times to get through all of them.
• For coherency with the other attribute errors, I didn't modify the default output too much, meaning that we have some new messages now. I'll likely come back to that to check if the previous ones were better in a case-by-case approach.
• doc(test(attr(...))) is handled in a horrifying manner currently. Until we can handle it correctly with the Attribute system, it'll remain that thing we're all very ashamed of. 😈
• Because of error reporting, some fields of Doc are suboptimal, like inline which instead of being an Option is a ThinVec because we report the error later on. Part of the things I'm not super happy about but can be postponed to future me.
• In src/librustdoc/clean/cfg.rs, the pub(crate) fn parse(cfg: &MetaItemInner) -> Result<Cfg, InvalidCfgError> { function should be removed once cfg_trace has been ported to new cfg API. https://github.com/rust-lang/rust/pull/150092
• Size of type DocFragment went from 32 to 48. Would be nice to get it back to 32.
• malformed `doc` attribute input wasn't meant for so many candidates, should be improved. https://github.com/rust-lang/rust/pull/149917
• See how many of the checks in check_attr we can move to attribute parsing
• Port target checking to be in the attribute parser completely
• Fix target checking for doc(alias) on fields & patterns
• See if we can use InvalidTarget instead of AttributeLintKind::AttrCrateLevelOnly and DocAttrNotCrateLevel