[wwwdocs] Add __cpp_* feature macros to C++20 entries that have those in projects/cxx_status.html

[Jakub Jelinek]

Hi!

I've noticed we don't have any feature test macros in the table for C++20,
even when a couple of the features have them defined.

Ok for wwwdocs?

--- htdocs/projects/cxx-status.html.jj  2019-01-11 14:24:06.524979265 +0100
+++ htdocs/projects/cxx-status.html     2019-01-11 16:41:16.650157887 +0100
@@ -148,7 +148,7 @@
        <a href="http://wg21.link/P0905r1";>P0905R1</a><br/>
        <a href="http://wg21.link/p1120r0";>P1120R0</a></td>
       <td class="unsupported"> No </td>
-      <td> </td>
+      <td> __cpp_impl_three_way_comparison &gt;= 201711 </td>
     </tr>
     <tr>
       <td> Access checking on specializations </td>
@@ -220,7 +220,7 @@
       <td> Class Types in Non-Type Template Parameters </td>
       <td><a href="http://wg21.link/p0732r2";>P0732R2</a></td>
       <td class="supported"> 9 </td>
-      <td> </td>
+      <td> __cpp_nontype_template_parameter_class &gt;= 201806 </td>
     </tr>
     <tr>
       <td> Atomic Compare-and-Exchange with Padding Bits </td>
@@ -232,7 +232,7 @@
       <td> Efficient sized delete for variable sized classes </td>
       <td><a href="http://wg21.link/p0722r3";>P0722R3</a></td>
       <td class="supported"> 9 </td>
-      <td> </td>
+      <td> __cpp_impl_destroying_delete &gt;= 201806 </td>
     </tr>
     <tr>
       <td> Allowing Virtual Function Calls in Constant Expressions </td>
@@ -257,7 +257,7 @@
       <td> explicit(bool) </td>
       <td><a href="http://wg21.link/p0892r2";>P0892R2</a></td>
       <td class="supported"> 9 </td>
-      <td> </td>
+      <td> __cpp_conditional_explicit &gt;= 201806 </td>
     </tr>
     <tr>
       <td> Signed integers are two's complement </td>
@@ -269,7 +269,7 @@
       <td> char8_t </td>
       <td><a href="http://wg21.link/p0482r6";>P0482R6</a></td>
       <td class="unsupported"> No </td>
-      <td> </td>
+      <td> __cpp_char8_t &gt;= 201811 </td>
     </tr>
     <tr>
       <td> Immediate functions (consteval) </td>

        Jakub