simplify the code for diagnostics severity adjustment

[Martin521]

Description

This PR simplifies the code for adjusting diagnostics severity, without change to the functionality. It will be needed for "scoped nowarn", where the adjustment logic becomes even more complex, but makes sense also in itself.

[github-actions[bot]]

:warning: Release notes required, but author opted out

[!WARNING] Author opted out of release notes, check is disabled for this pull request. cc @dotnet/fsharp-team-msft

[Martin521]

Can somebody add the "no release notes" label? Thanks

[psfinaki]

There you go, Martin :) Thanks for splitting the refactoring into a separate PR, that's always helpful.