Add support for P1144/P2786 trivial relocatability

[Quuxplusone]

Example: https://godbolt.org/z/bT1WPnY4z

[Quuxplusone]

@charles-salvia: Actually, I think now I spoke too soon. You've got this:

// -------- specialization for is_trivially_relocatable
//
template <class Y, class G, class D, class P>
struct is_trivially_relocatable<intrusive_ptr<Y,G,D,P>> : std::true_type
{ };

which doesn't work with a P1144-enabled standard library (because you're not allowed to specialize standard type traits). But I also don't yet understand why you need to specialize this for intrusive_ptr, along your pre-P1144 codepath — who's checking the value of this type-trait? I need to look deeper at this.