[libc++] Implement `operator<=>` for `thread::id`

The new operator<=> is mapped onto the existing functions
__libcpp_thread_id_equal and __libcpp_thread_id_less. Introducing a
new __libcpp_thread_id_compare_three_way might lead to more efficient
code. Given that we can still introduce __libcpp_thread_id_compare_three_way
later, for this commit I opted to not break ABI. If requested, I will
add __libcpp_thread_id_compare_three_way in a follow-up commit.

Implements part of P1614R2 "The Mothership has Landed"

Differential Revision: https://reviews.llvm.org/D131362