[llvm] [libc++] Fix CODEOWNERS file for libc++ (PR #65344)

[Louis Dionne via llvm-commits]

ldionne wrote:

> I wonder in general how many people want to subscribe to PRs, but don't want to be added as a reviewer

It's more about making sure that not just anyone can approve a patch on behalf of the library maintainers. It's useful to have a group representing "review authority" since it allows contributors to just submit patches and to know when they can be committed without having to know which individual reviews are required. Basically, whenever the code owner group is marked as "approved", the patch is good to go.

Conflating that with the group used for notifications means that we can't attach any meaning to "being approved by the codeowner group" anymore, and that's what we want to avoid (at least for the runtimes, I don't want to speak for other projects).

In that case, I think we want to create new teams:

```
reviewers-libcxxabi
reviewers-libunwind
```

And then I will change the code-owner file for these three projects to point to these groups. At that point, however, I don't know what `pr-subscribers-{libcxx,libcxxabi,libunwind}` will be useful for, since it won't get added on every review.

IMO it is more important to get this right than to be notified when the PR is created -- I might be wrong but I would think that being notified a few seconds after when the appropriate tag is added to the PR via Github Actions is enough?

https://github.com/llvm/llvm-project/pull/65344