Rollup merge of #143286 - Muscraft:track-diagnostics-note, r=WaffleLapkin

Make -Ztrack-diagnostics emit like a note

[#t-compiler/diagnostics > Rendering -Ztrack-diagnostics like a note](https://rust-lang.zulipchat.com/#narrow/channel/147480-t-compiler.2Fdiagnostics/topic/Rendering.20-Ztrack-diagnostics.20like.20a.20note/with/526608647)

As discussed on the Zulip thread above, I want to make `-Ztrack-diagnostics` emit like a `note`. This is because I find its current output jarring, and the fact that it gets rendered completely left-aligned, [even in the middle of a snippet](https://github.com/rust-lang/rust/blob/86e05cd300fac9e83e812c4d46582b48db780d8f/tests/ui/track-diagnostics/track6.stderr), seems like something that should be changed. Turning it into a `note` seems like the best choice, as it would align it with the rest of the output, and `note` is already used for somewhat similar things, like seeing why a lint was fired.

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Note: turning `-Ztrack-diagnostics` into a `note` will also make `annotate-snippets` API a bit cleaner

Author: matthiaskrgr