Rollup merge of #39820 - jonasbb:export-attributes, r=nrc
Export attributes in save-analysis data
Since this is my first pull-request to rust, I would like to get some feedback about obvious errors in this implementation.
I would like to change the save-analysis data to include arbitrary attribute data.
A use-case I have in mind for this is identifying functions with `#[test]` annotations such that tools like rls can offer a test-runner feature. I described my idea here [rls#173](https://github.com/rust-lang-nursery/rls/issues/173).
My changes contain:
1. track a vector of attributes in the various `*Data` types in `data.rs` and `external_data.rs`
2. implement lowering for `Attribute` and `MetaItem`
3. adjust `JsonDumper` to print the attributes
In the lowering of `Attribute` I remove the distinction between `MetaItem` and `NestedMetaItem`. I did this because this distinction is somewhat confusing. For example, `NestedMetaItemKind::Literal` has two identical spans, because both `NestedMetaItem` and `Lit` are defined as `Spanned<_>`.
My model is strictly more general, as it allows an `LitKind` instead of a `Symbol` for `MetaItem` and `Symbol`s are converted into a cooked string. As a consumer of the save-analysis data this shouldn't affect you much.