[libc++] Fix a debug-hardening test that requires C++17

We only support <filesystem> starting with C++17.

Author: ldionne

libcxx/test/libcxx/input.output/filesystems/class.path/path.itr/assert.iterator.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-unix-headers
-// UNSUPPORTED: c++03
+// UNSUPPORTED: c++03, c++11, c++14
 // UNSUPPORTED: !libcpp-hardening-mode=debug
 // XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing