This saves the storage space used by about 32 bits per `Fingerprint`.
On average, this reduces the size of the `/target/{mode}/incremental`
folder by roughly 5%.

Fixes #45875

incr.comp.: Load diagnostics from previous session lazily and clean up on-disk-cache persistence code.

The previous method ran into problems because ICH would treat Spans
as (file,line,col) but the cache contained byte offsets and its
possible for the latter to change while the former stayed stable.

incr.comp.: Remove default serialization implementations for things in rustc::hir::def_id so that we get an ICE instead of silently doing the wrong thing.

incr.comp.: Store the query result index which records where query results can be found in the cached.

incr.comp.: Allow for mapping from prev-session-CrateNums to current-session-CrateNums in OnDiskCaches.

incr.comp.: Include header when loading cache files in order to get the same byte offsets as when saving.