internal: Improve rust-analyzer diagnostics

[ChayimFriedman2]

In my experience the processing <module> messages make it harder to search for the actual diagnostics, so remove them and instead print the filename only if there is a diagnostic.

Also allow choosing the minimum severity.

[lnicola]

Should we use a progress bar like in analysis-stats? The file names are useful there to track down panics.

[ChayimFriedman2]

I don't think our progress bar supports multiple (that is, not overriding-the-previous-) prints, and overall I think the current situation is worse. Panic info we can get from analysis-stats, when I run diagnostics I want to get, well, diagnostics, and in my experience the current interface is pretty unusable.

[lnicola]

I don't think our progress bar supports multiple (that is, not overriding-the-previous-) prints

I think we use println! for the diagnostics and the progress bar for the files. But feel free to merge, it's not too important right now.

[ChayimFriedman2]

That's what I said, I think each println will override the previous one.

[lnicola]

No, I think that's how we print type mismatches in analysis-stats -v.

[ChayimFriedman2]

Ooh cool - I'll switch to that then.

[ChayimFriedman2]

@lnicola Changed the PR do use a progress bar.