Fix error in file_and_directory patch

Fix error in file_and_directory patch

In my earlier C++-ization patch for file_and_directory, I introduced
an error:

-  if (strcmp (fnd.name, "<unknown>") != 0)
+  if (fnd.is_unknown ())

This change inverted the sense of the test, which causes failures with
.debug_names.

This patch fixes the bug.  Regression tested on x86-64 Fedora 34.  I
also tested it using the AdaCore internal test suite, with
.debug_names -- this was failing before, and now it works.