Go to next/previous diagnostic skipped every diagnostic tagged as
unnecessary, while the editor only drops the underline for unnecessary
diagnostics below warning severity. An error that is also tagged as
unnecessary, like an unused variable in Go, was shown with a red
squiggle but F8 jumped right past it.
Only skip unnecessary diagnostics that are faded out without an
underline, matching how they are rendered.
Closes#64790
dualfroz
upstream