Implement LWG 2686, std::hash<error_condition>, for C++17

Implement LWG 2686, std::hash<error_condition>, for C++17

2017-03-23  Daniel Kruegler  <daniel.kruegler@gmail.com>

	Implement LWG 2686, Why is std::hash specialized for error_code,
	but not error_condition?
	* include/std/system_error (hash<error_condition>): Define for C++17.
	* testsuite/20_util/hash/operators/size_t.cc (hash<error_condition>):
	Instantiate test for error_condition.
	* testsuite/20_util/hash/requirements/explicit_instantiation.cc
	(hash<error_condition>): Instantiate hash<error_condition>.

From-SVN: r246424