[libc++] [compare] Named comparison functions, is_eq etc.

Some of these were previously half-implemented in "ordering.h";
now they're all implemented, and tested.
Note that `constexpr` functions are implicitly `inline`, so the
standard wording omits `inline` on these; but Louis and I agree
that that's surprising and it's better to be explicit about it.

Differential Revision: https://reviews.llvm.org/D110515