Define [range.cmp] comparisons for C++20

Define [range.cmp] comparisons for C++20

Define std::identity, std::ranges::equal_to, std::ranges::not_equal_to,
std::ranges::greater, std::ranges::less, std::ranges::greater_equal and
std::ranges::less_equal.

	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/range_cmp.h: New header for C++20 function objects.
	* include/std/functional: Include new header.
	* testsuite/20_util/function_objects/identity/1.cc: New test.
	* testsuite/20_util/function_objects/range.cmp/equal_to.cc: New test.
	* testsuite/20_util/function_objects/range.cmp/greater.cc: New test.
	* testsuite/20_util/function_objects/range.cmp/greater_equal.cc: New
	test.
	* testsuite/20_util/function_objects/range.cmp/less.cc: New test.
	* testsuite/20_util/function_objects/range.cmp/less_equal.cc: New test.
	* testsuite/20_util/function_objects/range.cmp/not_equal_to.cc: New
	test.

From-SVN: r277120