Rollup merge of #117207 - Zalathar:no-option, r=compiler-errors

The value of `-Cinstrument-coverage=` doesn't need to be `Option`

(Extracted from #117199, since this is a purely internal cleanup that can land independently.)

Not using this flag is identical to passing `-Cinstrument-coverage=off`, so there's no need to distinguish between `None` and `Some(Off)`.