[libc++] Mark [P0475] "LWG2511: guaranteed copy elision for piecewise construction" as Complete.

The point of LWG2511 is basically just to make sure that we use
`tuple<Args&&...>` instead of `tuple<Args...>` in a couple of places
inside `scoped_allocator_adaptor` and inside `pair`.
As far as I can tell, this has been true for libc++
since EricWF's D27612 (and maybe even earlier than that).