Allow <filesystem> within Drake

Author: jwnimmer-tri

cppguide.html

@@ -4369,11 +4369,16 @@ <h3 id="Other_Features"><a name="C++11">Other C++ Features</a></h3>
   <code>&lt;fenv.h&gt;</code> headers, because many
   compilers do not support those features reliably.</li>
 
+<span class="nondrake">
   <li>The <code>&lt;filesystem&gt;</code> header, which
 
   does not have sufficient support for testing, and suffers
   from inherent security vulnerabilities.</li>
-
+</span>
+<span class="drake">
+  <li>The <code>&lt;filesystem&gt;</code> header, until it is available on all
+  of our supported platforms.</li>
+</span>
 
 </ul>