[libc++] [D2358R0] ADL-proof `projected`

[• Quuxplusone]

Summary

This implements D2538R0 "ADL-proof std::projected", just to prove
that it works.

Notice that because Holder<Incomplete> is _possible_ to complete,
but _unsafe_ to complete, that means that Holder<Incomplete>*
is basically not an iterator and it's not even safe to ask if
input_iterator<Holder<Incomplete>*> because that _will_ necessarily
complete the type. So it's totally expected that we still cannot
safely ask e.g.

static_assert(std::indirect_unary_predicate<bool(&)(Holder<Incomplete>&), Holder<Incomplete>*>);

or even

static_assert(!std::indirect_unary_predicate<int, Holder<Incomplete>*>);