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

[Tom Stellard via llvm-commits]

tstellar wrote:

> Hmm, I just saw your post here: https://discourse.llvm.org/t/changes-to-pull-request-subscription-system/73296/15.

I think it's OK for the libcxx reviewers to use the file now, since it provide similar functionality to what you were using in Phabricator.  I just don't want ad hoc updates to the file without a plan.

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