Auto merge of #114669 - cjgillot:metadata-wp, r=petrochenkov

Make metadata a workproduct and reuse it

This PR aims to skip the generation of metadata by reusing the infrastructure that already exists for compiled codegen-units, namely "workproducts".

This can yield substantial gains (~10%) when we can demonstrate that metadata does not change between an incremental session and the next. This is the case if the crate is unchanged, or if all the changes are in upstream crates and have no effect on it. This latter case is most interesting, as it arises regularly for users with several crates in their workspace.

TODO:
- [x] Materialize the fact that metadata encoding relies on the relative order of definitions;
- [x] Refactor the handling of doc links.

Author: bors