cxx_stdcxxfs: Instantiate more code in the linking test

clang requires more instantiations than we currently have to exercise
their filesystem library.

nc_cxx_stdcxxfs.cpp

@@ -1,4 +1,5 @@
 #include <filesystem>
+#include <iostream>
 
 int
 main (int argc, char **argv)
@@ -6,6 +7,9 @@ main (int argc, char **argv)
     (void)argc;
     (void)argv;
 
+    // Instantiating a path isn't enough for clang. We need to call a method
+    // implemented in the external library; like 'stem'.
     std::filesystem::path p { "foo" };
+    std::cout << p.stem () << '\n';
     return 0;
 }