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

[Hubert Tong via cfe-commits]

================
@@ -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 'A module directive may only appear as
+            the first preprocessing tokens in a file' was supported
+            in clang-21.
----------------
hubert-reinterpretcast wrote:

Note: Clang versions are referenced as "Clang <number>" (and not as clang-<number>) elsewhere in this file.
```suggestion
            The restriction that "[a] module directive may only appear
            as the first preprocessing tokens in a file" is enforced
            starting in Clang 21.
```

https://github.com/llvm/llvm-project/pull/164095