[Bug libstdc++/60271] [C++1y] std::max(initializer_list<T>) cannot use std::max_element

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60271

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Even if max_element() isn't made constexpr, we can make __max_element constexpr
(once the FE supports it) and use it directly in max.

It's certainly solvable, I just wanted to create this PR so we don't forget
about it.