Implement std::common_reference for C++20

Implement std::common_reference for C++20

	* include/std/type_traits (__do_common_type_impl): Implement
	additional COND-RES(CREF(D1), CRED(D2)) condition for C++20.
	(basic_common_reference, common_reference, common_reference_t): Define
	for C++20.
	* testsuite/20_util/common_reference/requirements/alias_decl.cc: New
	test.
	* testsuite/20_util/common_reference/requirements/
	explicit_instantiation.cc: New test.
	* testsuite/20_util/common_reference/requirements/typedefs.cc: New
	test.

From-SVN: r275594