rustdoc-json: Structured attributes
Implements and closesrust-lang/rust#141358.
This has 2 primary benefits.
1. For rustdoc-json consumers, they no longer need to parse strings of attributes, but it's there in a structured and normalized way. CC ```@obi1kenobi```
2. For rustc conributors, the output of HIR pretty printing is no longer a versioned thing in the output. People can work on rust-lang/rust#131229 without needing to bump `FORMAT_VERSION`. CC ```@jdonszelmann``` ```@JonathanBrouwer.```
(Over time, as the attribute refractor continues, I expect we'll add new things to `rustdoc_json_types::Attribute`. But this can be done separately to the rustc changes).
Todo before being mergable:
- [x] Update test assertions.
- [x] Fix modeling of `#[repr]`.
- [ ] ~~Add tests of `#[doc(hidden)]` in `Item::attrs` (probably in a seperate PR).~~ I'm gonna punt this to a future PR
- [x] Documentation.
9f2015d3d3