[libc++] Mark [P0809] "LWG2831: Comparing Unordered Containers" as Nothing To Do.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0809r0.pdf

This issue/paper simply removed some library UB because vendors were
already doing the right thing. libc++ has always done the right thing
(in this respect).

Differential Revision: https://reviews.llvm.org/D93816