HirIdification: rework Map

The next iteration of HirIdification (#57578).

- remove `NodeId` from `Entry`
- change `Map::map` to an `FxHashMap<HirId, Entry>`
- base the `NodeId` `Map` methods on `HirId` ones (reverses the current state)
- HirIdify `librustdoc` a little bit (some `NodeId` `Map` methods were converted to work on `HirId`s)

The second change might have performance implications, so I'd do a perf run to be sure it's fine; it simplifies the codebase and shouldn't have an impact as long as the `Map` searches are cached (which is now possible thanks to using `HirId`s).

r? @Zoxc