Add garbage collect of diagnostics

[ndmitchell]

Currently if you have a file open with errors, then shut it, the errors persist. The reason is that we don't have any sophisticated removal of non-interesting files, unlike what we have with DAML.

[pepeiborra]

I think that we want to at least keep the errors for dependencies of the current files of interest, those are very helpful. And I think that @mpickering may want a completely different behaviour here, as he typechecks all the targets at startup.

In implementation terms, we want to garbage collecting diagnostics which do not belong to the set of dependencies of the files of interest. This should happen after kick is done. There is some logic to do this already in the repo, but it is not being used and likely to be deleted as dead code and lost:
https://github.com/digital-asset/ghcide/blob/e16e841fa7e51b7950fd309196c1b16e51f95983/src/Development/IDE/Core/Shake.hs#L448-L462

[mpickering]

garbageCollect should run when a file is deleted from the FOI HashSet, not on every kick in my opinion. (As in my opinion, nothing should run in kick as it adds costs to every call).

[ndmitchell]

I agree very little should run in kick, but if you have a file of interest A.hs which imports B.hs, then edit A.hs so it no longer imports B.hs, then any warnings shown as a result of B.hs should disappear if it's not included by another other root. In DAML we adjusted that logic, so only if A.hs parsed successfully did we trim B.hs, to avoid losing warnings on every parse error.

[jneira]

This continue happening with ghcide and hls master