[libc++] Hash-pin or version libcxx/utils/requirements.txt

It would be nice if we can hash/version pin this, but that's a separate patch given this is used elsewhere.

_Originally posted by @boomanaiden154 in https://github.com/llvm/llvm-project/pull/212859#discussion_r3754150015_

Start with libcxx/utils/requirements.txt and inspect where it is used elsewhere, as noted in the issue. Determine the appropriate versions or hashes for the listed requirements, then verify that the file remains usable by its consumers; done means every relevant requirement is pinned or hash-pinned without breaking those uses.

Written by the indexing model from the issue text.