[parallel-queries] Refactor error message handling to better integrate with parallel queries

[michaelwoerister]

At the moment error message handling uses a callback-based setup in order to handle diagnostics tracking during query evaluation. This could probably be handled in a cleaner way.

Also, the current system has no way of making error message output somewhat deterministic under parallel execution.

Both problems should be solved by re-factoring the system to better fit the changed needs.

[sasurau4]

It seems that this issue solved with #67614.

[michaelwoerister]

I don't think that PR takes care of emitting error messages in a deterministic order, right?

[Zoxc]

@rustbot label WG-compiler-parallel

[Zoxc]

I'm not sure we'd want to do this is it would require error messages to be delayed.