[NFC][doc] Mark P1857R3 as partial implemented (#164095)

Since the following 2 patches was landed, mark P1857R3 as partial
implemented.

- [[C++][Modules] A module directive may only appear as the first
preprocessing tokens in a
file](#144233).
- [[clang] Allow trivial pp-directives before C++ module
directive](#153641).

---------

Signed-off-by: yronglin <[email protected]>
Co-authored-by: h-vetinari <[email protected]>
Co-authored-by: Hubert Tong <[email protected]>

Author: 3 people

clang/www/cxx_status.html

@@ -926,7 +926,14 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p1857r3">P1857R3</a></td>
-        <td class="none" align="center">No</td>
+        <td class="partial" align="center">
+          <details>
+            <summary>Clang 21 (Partial)</summary>
+            The restriction that "[a] module directive may only appear
+            as the first preprocessing tokens in a file" is enforced
+            starting in Clang 21.
+          </details>
+        </td>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p2115r0">P2115R0</a></td>