Implement C++17 constexpr if.

Implement C++17 constexpr if.

	* cp-tree.h (IF_STMT_CONSTEXPR_P): New.
	* name-lookup.c (push_to_top_level, pop_from_top_level_1): Handle it.
	* parser.h (struct cp_parser): Add in_discarded_stmt field.
	* parser.c (cp_parser_selection_statement): Handle 'if constexpr'.
	(cp_parser_jump_statement): Avoid deducing from a discarded return.
	* pt.c (tsubst_expr): Only instantiate taken branch of constexpr if.
	* semantics.c (begin_if_stmt): Set the binding level this_entity.
	(finish_if_stmt_cond): Require the condition of a
	constexpr if to be constant.
	* decl.c (level_for_constexpr_if): New.
	(named_label_entry): Add in_constexpr_if field.
	(poplevel_named_label_1): Set it.
	(check_goto): Check it.
	(check_previous_goto_1): Check level_for_constexpr_if.

From-SVN: r239338