c++: Do not warn about lifetime of std::initializer_list<T>& [PR102482]

c++: Do not warn about lifetime of std::initializer_list<T>& [PR102482]

An initializer-list constructor taking a non-const lvalue cannot be
called with a temporary, so the array's lifetime probably doesn't end
with the full expression. -Winit-list-lifetime should not warn for that
case.

	PR c++/102482

gcc/cp/ChangeLog:

	* init.c (maybe_warn_list_ctor): Do not warn for a reference to
	a non-const std::initializer_list.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Winit-list5.C: New test.