[Bug libstdc++/87106] Group move and destruction of the source, where possible, for speed

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87106

--- Comment #26 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Or maybe that should be:

  template<typename _Tp, typename = void>
    struct __is_bitwise_relocatable
    : is_trivially_copyable<_Tp> { };

...

          if constexpr (!is_trivial_v<T>)
            if (is_default_constructible_v<_Tp> && (__OPTIMIZE__+0))
              std::__uninitialized_default_novalue_n(result, result + count);
            else
              {
                // can't do bitwise relocation today
                ...
              }