Implement P0258R2 - helper for C++17
	std::has_unique_object_representations trait
c-family/
	* c-common.h (enum rid): Add RID_HAS_UNIQUE_OBJ_REPRESENTATIONS.
	* c-common.c (c_common_reswords): Add
	__has_unique_object_representations.
cp/
	* cp-tree.h (enum cp_trait_kind): Add
	CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS.
	(struct lang_type_class): Add unique_obj_representations
	and unique_obj_representations_set bitfields.
	(CLASSTYPE_UNIQUE_OBJ_REPRESENTATIONS,
	CLASSTYPE_UNIQUE_OBJ_REPRESENTATIONS_SET): Define.
	(type_has_unique_obj_representations): Declare.
	* parser.c (cp_parser_primary_expression): Handle
	RID_HAS_UNIQUE_OBJ_REPRESENTATIONS.
	(cp_parser_trait_expr): Likewise.  Formatting fix.
	* semantics.c (trait_expr_value, finish_trait_expr): Handle
	CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS.
	* tree.c (type_has_unique_obj_representations): New function.
	(record_has_unique_obj_representations): New function.
	* cxx-pretty-print.c (pp_cxx_trait_expression): Handle
	CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS.
testsuite/
	* g++.dg/cpp1z/has-unique-obj-representations1.C: New test.
	* g++.dg/cpp1z/has-unique-obj-representations2.C: New test.

From-SVN: r240843