Implements https://www.github.com/rust-lang/rust/issues/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.
2. For rustc contributors, the output of HIR pretty printing is no
longer a versioned thing in the output. People can work on
https://github.com/rust-lang/rust/issues/131229 without needing to
bump `FORMAT_VERSION`.
(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).
