[libc++] Implement `thread::id` comparators as free functions

So far, the `thread::id` comparators were implemented as hidden friends.
This was non-conforming and lead to incorrectly rejected C++ code, as
can be seen in the linked Github issue.

Fixes https://github.com/llvm/llvm-project/issues/56187

Differential Revision: https://reviews.llvm.org/D131430