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

[Louis Dionne via llvm-commits]

ldionne wrote:

> CODEOWNERS is encoding 1) and not 2) right now

I don't think that is correct, or at least it might be the intent but not really what's actually happening. For example, see https://github.com/llvm/llvm-project/pull/65279:

<img width="327" alt="Screenshot 2023-09-05 at 4 56 17 PM" src="https://github.com/llvm/llvm-project/assets/700834/a1cd333f-7e6b-4c79-849b-2026dd9a8b76">

It looks like the @llvm/pr-subscribers-libcxx group was added automatically as a reviewer of that PR.

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