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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> > 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?
> 
> The motivation was to have a "good" notification, that is an email which is directly actionable and not "just a link", which led us to the CODE_OWNERS file. But Tom figured out some magic to get this done now!
> 
> I'd note that LLVM as a project does not have concept of code owners required approval, commit access is technically enough to review and push code, and folks are supposed to apply best judgement with respect to their level of familiarity with parts of the codebase! I'm not sure there is another doc for the project than https://llvm.org/docs/CodeReview.html ?

libc++/libc++abi/libunwind have had blocking reviewer group on Phabricator for a long time, since March 2020:
https://discourse.llvm.org/t/new-herald-rules-to-ease-libc-and-libc-abi-reviews/54727
(libunwind may be slightly later). I agree that for all other components, I haven't heard of a blocking-reviewer-by-default rule.

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