[libc++] Do not require movability in __non_propagating_cache::__emplace_deref

As explained in http://eel.is/c++draft/range.nonprop.cache#note-1, we
should allow copy and move elision to happen when calling emplace_deref
in non-propagating-cache. Before this change, the only way to emplace
into the non-propagating-cache was to call `__set(*it)`, which materialized
`*it` when binding it to the reference argument of `__set` and disabled
move elision.

As a fly-by change, this also renames `__set` to `__emplace` for consistency
and adds tests for it.

Differential Revision: https://reviews.llvm.org/D107932