I put this in its own commit in case all of the information removed here
was controversial. But it *looks* stale to me. At the very least,
`TypeCheckDiagnostic` no longer exists, so that would need to be fixed.
And it doesn't really make sense to me (at this point) to make
`Diagnostic` a Salsa struct, particularly since we are keen on using it
in Ruff (at some point).
185bcfef1a