Make flycheck diagnostics track their generation

[Veykril]

Noting this down before I forget again. This was sparked by a RustWeek discussion.

We should make flycheck diagnostics generational after all. Why? Because there seem to be setups where cargo may not report an artifact to our runner at all, causing us to not record an event for a crate which ultimately may result in us not clearing diagnostics in a given crate. This has manifested a couple times for me already, in part getting so bad that there are diagnostics that never get cleared at all. This is obviously not great.

So the idea would be to clear all diagnostics of a differing generation than the current if flycheck completes successfully.