PR c++/47207
	* decl2.c (decl_constant_var_p): A constexpr var needs an
	initializer to be constant.
	* semantics.c (cxx_eval_constant_expression): Complain about
	constexpr var used in its own initializer.
	* call.c (set_up_extended_ref_temp): Set
	DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P too.

From-SVN: r170365