Restructure metadata encoder to track deps precisely
This issue restructures meta-data encoding to track dependencies very precisely. It uses a cute technique I hope to spread elsewhere, where we can guard the data flowing into a new dep-graph task and ensure that it is not "leaking" information from the outside, which would result in missing edges. There are no tests because we don't know of any bugs in the old system, but it's clear that there was leaked data.
The commit series is standalone, but the refactorings are kind of "windy". It's a good idea to read the comment in `src/librustc_metadata/index_builder.rs` to get a feeling for the overall strategy I was aiming at.
In several cases, I wound up adding some extra hashtable lookups, I think primarily for looking up `AdtDef` instances. We could remove these by implementing `DepGraphRead` for an `AdtDef` and then having it register a read to the adt-defs table, I guess, but I doubt it is really noticeable.
Eventually I think I'd like to extend this pattern to the dep-graph more generally, since it effectively guarantees that data cannot leak.
Fixes#35111.
r? @michaelwoerister
499484f56d