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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

The principal motivation seems to be "requires new committers to know what individuals to ping to get a review", and "make it easier for new committers to get involved (they don't need to fill the Reviewers: field anymore)", which is fairly common across the project:  not clear to me that "blocking reviewer" is necessary over simply setting up "reviewer" though (Herald allows to setup "reviewers" without blocking anything, which is how I setup myself for most of the areas of MLIR where I am "code owner" (and I intend to continue to do so with CODE_OWNERS on GitHub to address exactly this problem of people who need to figure out who to ask a review from!)

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