PR c++/58511
	* semantics.c (is_instantiation_of_constexpr): Return true for
	defaulted functions, too.
	(explain_invalid_constexpr_fn): Only use
	explain_implicit_non_constexpr if !DECL_DECLARED_CONSTEXPR_P.
	* method.c (explain_implicit_non_constexpr): Pass
	DECL_INHERITED_CTOR_BASE to explain_implicit_non_constexpr.

From-SVN: r212507