Support [@coverage off] for or-patterns

[nlsandler]

You can use [@coverage off] to exclude pattern-matching cases, but this doesn't work for or-patterns. Here's a minimal example:

I would expect the case

| B | C -> (1 [@coverage off])

to be excluded from code coverage checking, the same as if both cases appeared individually or in a wildcard pattern. Issue #246 mentions that [@coverage off] should be able to exclude or-patterns.

I'm using OCaml version 4.14.1, dune version 3.7.1, bisect_ppx version 2.8.2.