The hash value should be based on the identity (i.e. address) of the
error_category member, not its object representation (i.e. underlying
bytes).

	* include/std/system_error (error_code): Remove friend declaration
	for hash<error_code>.
	(hash<error_code>::operator()): Use public member functions to access
	value and category.
	(hash<error_condition>::operator()): Use address of category, not
	its object representation.
	* src/c++11/compatibility-c++0x.cc (hash<error_code>::operator()):
	Use public member functions to access value and category.
	* testsuite/19_diagnostics/error_condition/hash.cc: New test.

From-SVN: r270872