Check for overflow in filesystem::last_write_time

	* include/experimental/bits/fs_fwd.h (file_time_type): Simplify
	definition.
	* src/filesystem/ops.cc (file_time): Take error_code parameter and
	check for overflow.
	(do_copy_file, last_write_time): Pass error_code in file_time calls.
	* testsuite/experimental/filesystem/operations/last_write_time.cc:
	New.
	* testsuite/util/testsuite_fs.h (scoped_file): Define RAII helper.

From-SVN: r240587