Add tests for assert contract attributes.
	* g++.dg/cpp2a/contracts1.C: Fix existing tests and add details.
	* g++.dg/cpp2a/contracts2.C: Fix existing tests and add details.
	* g++.dg/cpp2a/contracts3.C: New.
	* g++.dg/cpp2a/contracts4.C: New.
	* g++.dg/cpp2a/contracts5.C: New.
	* g++.dg/cpp2a/contracts6.C: New.
	* g++.dg/cpp2a/contracts7.C: New.
	* g++.dg/cpp2a/contracts8.C: New.
	* g++.dg/cpp2a/contracts9.C: New.

2018-08-13  Jeff Chapman II  <jchapman@lock3software.com>

	Add tests for assert contract attributes.
	* src/c++17/contract.cc (handle_contract_violation): Print a
	testing string to stderr whenever called for testing.

2018-08-13  Jeff Chapman II  <jchapman@lock3software.com>

	Add example of a custom contract violation handler
	* g++.dg/cpp2a/preload_handler/Makefile: New.
	* g++.dg/cpp2a/preload_handler/README: New.
	* g++.dg/cpp2a/preload_handler/assert_fail.cpp: New.
	* g++.dg/cpp2a/preload_handler/handle_contract_violation.cpp: New.