Implement N3653 (Member initializers and aggregates) and fix references to 'this' in constexpr constructors.

	Implement N3653 (Member initializers and aggregates) and fix
	references to 'this' in constexpr constructors.
	* class.c (check_field_decls): In C++14 an NSDMI does not make the
	class non-aggregate.
	* constexpr.c (struct constexpr_ctx): New.
	(cxx_bind_parameters_in_call): Handle 'this'.
	(cxx_eval_call_expression): Create new constexpr_ctx.
	(cxx_eval_component_reference): Check CONSTRUCTOR_NO_IMPLICIT_ZERO.
	(initialized_type, init_subob_ctx, verify_ctor_sanity): New.
	(cxx_eval_bare_aggregate): Use them.  Build CONSTRUCTOR early.
	(cxx_eval_vec_init_1): Likewise.
	(cxx_eval_constant_expression) [PARM_DECL]: Allow 'this'.
	[TARGET_EXPR]: Build new constexpr_ctx.
	[PLACEHOLDER_EXPR]: New.
	(cxx_eval_outermost_constant_expr): Build new constexpr_ctx.  Add
	object parameter.
	(is_sub_constant_expr): Build new constexpr_ctx.
	(potential_constant_expression_1): Handle PLACEHOLDER_EXPR.
	Allow 'this'.
	* cp-gimplify.c (cp_gimplify_init_expr): Call replace_placeholders.
	* cp-tree.h (CONSTRUCTOR_NO_IMPLICIT_ZERO): New.
	* error.c (dump_expr): Handle PLACEHOLDER_EXPR.
	* init.c (get_nsdmi): Generate PLACEHOLDER_EXPR.
	* tree.c (lvalue_kind): Handle PLACEHOLDER_EXPR.
	(build_ctor_subob_ref, replace_placeholders): New.
	* typeck2.c (store_init_value): Use replace_placeholders.
	(process_init_constructor_record): Make zero-init before NSDMI
	explicit.

From-SVN: r216750