One of the bullets in 9.5.5 [dcl.init.list] paragraph 3 says,

Otherwise, if T is a specialization of std::initializer_list<E>, an initializer_list object is constructed as described below and used to initialize the object according to the rules for initialization of an object from a class of the same type (9.5 [dcl.init]).

This does not, but should, say whether the initializer_list object is treated as an lvalue or prvalue for the purpose of the 9.5 [dcl.init] initialization.