Update: The situation got a bit more complicated after the resolution of CWG DR 1684 removed the requirement quoted below. Some more information can be found in this discussion on the std-discussion mailing list and in the related question Why isn’t `std::initializer_list` defined as a literal type?
[decl.constexpr]/8:
A constexpr specifier for a non-static member function that is not a constructor declares that member function to be const (9.3.1). […] The class of which that function is a member shall be a literal type (3.9).
Therefore, the changes of N3471 guarantee std::initializer_list will be a literal type.
Note the constexpr ctor alone doesn’t require std::initializer_list to be a literal type, see [dcl.constexpr]/4+8. Side note: An object of non-literal type with constexpr ctor can be initialized during constant initialization [basic.start.init]/2] (part of static initialization, performed before any dynamic initialization).