[cxx_status] Mark support for std::is_constant_evaluated as done.

Eric implemented this in r359067.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@360310 91177308-0d34-0410-b5e6-96231b3b80d8

www/cxx_status.html

@@ -1023,7 +1023,7 @@ as the draft C++2a standard evolves.
     <tr>
       <td><tt>std::is_constant_evaluated</tt></td>
       <td><a href="http://wg21.link/p0595r2">P0595R2</a></td>
-      <td class="none" align="center">No</td>
+      <td class="svn" align="center">SVN</td>
     </tr>
     <tr>
       <td>Nested inline namespaces</td>