This adds rangified overloads for for_each_n, sample and clamp as per P1243R4.

libstdc++-v3/ChangeLog:

	P1243R4 Rangify new algorithms
	* include/bits/ranges_algo.h (for_each_n_result, __for_each_n_fn,
	for_each_n, __sample_fn, sample, __clamp_fn, clamp): New.
	* testsuite/25_algorithms/clamp/constrained.cc: New test.
	* testsuite/25_algorithms/for_each/constrained.cc: Augment test.
	* testsuite/25_algorithms/sample/constrained.cc: New test.