Goal:

• Diagnostic enums should contain the (salsa) ids of the things involved in the error (eg the TyIds of the types for a type mismatch). These enum variants should generally be constructed directly, rather than via constructor functions that might generate strings, etc.
• The complexity of constructing helpful messages/labels for the user should be consolidated in the to_complete(..) -> CompleteDiagnostic functions.
• We should add convenience functions to the (salsa) id types where it's helpful.

I scrapped the LazyDiagnostic idea that I showed during the meeting, in favor of just moving the diagnostic stuff from the hir crate to the hir-analysis crate, so now DiagnosticVoucher::to_complete takes a SpannedHirAnalysisDb instead of a SpannedHirDb, etc. Its useful to have concrete span locations when generating the diagnostic text, so we can sort the labels by location and use text like "x defined first here", "x redefined here", etc and have it make sense.

I'll submit some follow-up PRs that actually improve diagnostics in the future, and to do a little more code shuffling. I'd like to merge this because it touches a lot of lines and could lead to conflict hassles for other stuff.