Problem: unnecessary and deprecated diagnostics use their own highlight
groups (`DiagnosticUnnecessary` and `DiagnosticDeprecated`) which
override the typical severity-based highlight groups (like
`DiagnosticUnderlineWarn`).
This can be misleading, since diagnostics about unused variables which
are warnings or errors, are shown like comments, since then only the
`DiagnosticUnnecessary` highlight group is used. Users do not see the
more eye-catching red/yellow highlight.
Solution: Instead of overriding the highlight group to
`DiagnosticUnnecessary` or `DiagnosticDeprecated`, set them in addition
to the normal severity-based highlights.
2767eac320