Unused header warning and `modernize-use-override` warnings.
I would like to selectively disable warnings about redundant `virtual` and
`override` keywords, since both together are useful (`override` enables
important compiler warnings that can avoid bugs, `virtual` helps quickly
identifying API functions available for overriding). But this doesn't
seem to be possible, only all of `modernize-use-override` can be
disabled, so simply keep the `virtual` but comment it out.
add3ec71d2