[Bug libstdc++/108619] Compilation error if the construct method of the allocator isn't implemented and the detructor of value_type is private

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108619

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And it would be nice to simplify allocator_traits::construct using if-constexpr
too.