PR libstdc++/90557 fix path assignment that alters source

PR libstdc++/90557 fix path assignment that alters source

Backport from mainline
2019-05-22  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/90557
	* src/c++17/fs_path.cc (path::_List::operator=(const _List&)): Fix
	reversed arguments to uninitialized_copy_n.
	* testsuite/27_io/filesystem/path/assign/copy.cc: Check that source
	is unchanged by copy assignment.
	* testsuite/util/testsuite_fs.h (compare_paths): Use std::equal to
	compare path components.

From-SVN: r271528