libstdc++: Implement "Safe Integral Comparisons" (P0586R2)

libstdc++: Implement "Safe Integral Comparisons" (P0586R2)

	* include/std/type_traits (__is_standard_integer): New helper trait.
	* include/std/utility (cmp_equal, cmp_not_equal, cmp_less, cmp_greater)
	(cmp_less_equal, cmp_greater_equal, in_range): Define for C++20.
	* include/std/version (__cpp_lib_integer_comparison_functions): Define.
	* testsuite/20_util/integer_comparisons/1.cc: New test.
	* testsuite/20_util/integer_comparisons/2.cc: New test.
	* testsuite/20_util/integer_comparisons/equal.cc: New test.
	* testsuite/20_util/integer_comparisons/equal_neg.cc: New test.
	* testsuite/20_util/integer_comparisons/greater_equal.cc: New test.
	* testsuite/20_util/integer_comparisons/greater_equal_neg.cc: New test.
	* testsuite/20_util/integer_comparisons/greater_neg.cc: New test.
	* testsuite/20_util/integer_comparisons/in_range.cc: New test.
	* testsuite/20_util/integer_comparisons/in_range_neg.cc: New test.
	* testsuite/20_util/integer_comparisons/less.cc: New test.
	* testsuite/20_util/integer_comparisons/less_equal.cc: New test.
	* testsuite/20_util/integer_comparisons/less_equal_neg.cc: New test.
	* testsuite/20_util/integer_comparisons/less_neg.cc: New test.
	* testsuite/20_util/integer_comparisons/not_equal.cc: New test.
	* testsuite/20_util/integer_comparisons/not_equal_neg.cc: New test.