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

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

	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.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271527 138bc75d-0d04-0410-961f-82ee72b054a4