Implement C++17 P0025 clamp.

Implement C++17 P0025 clamp.

2016-07-15  Edward Smith-Rowland  <3dw4rd@verizon.net>

	Implement C++17 P0025 clamp.
	* include/bits/algorithmfwd.h: Declare clamp overloads.
	* include/bits/stl_algo.h: Implement clamp.  Feature __cpp_lib_clamp.
	* testsuite/25_algorithms/clamp/1.cc: New test.
	* testsuite/25_algorithms/clamp/2.cc: New test.
	* testsuite/25_algorithms/clamp/constexpr.cc: New test.
	* testsuite/25_algorithms/clamp/requirements/explicit_instantiation/
	1.cc: New test.
	* testsuite/25_algorithms/clamp/requirements/explicit_instantiation/
	pod.cc: New test.

From-SVN: r238383