[NFC] [C++] [Modules] Mark P2788 as implemented and add test (#147138)

This is already implemented.

Author: kish1n

clang/test/SemaCXX/P2788.cpp

@@ -0,0 +1,17 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+
+// RUN: %clang_cc1 -std=c++20 -verify -emit-module-interface %t/B.cpp -o %t/B.pcm
+// RUN: %clang_cc1 -std=c++20 -verify -emit-module-interface %t/A.cpp -fmodule-file=A:B=%t/B.pcm -o %t/A.pcm
+
+//--- A.cpp
+// expected-no-diagnostics
+export module A;
+import :B;
+export int x = dimensions + 1;
+
+//--- B.cpp
+// expected-no-diagnostics
+export module A:B;
+const int dimensions=3;

clang/www/cxx_status.html

@@ -947,7 +947,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr> <!-- from Issaquah 2023 -->
         <td><a href="https://wg21.link/P2788R0">P2788R0</a> (<a href="#dr">DR</a>)</td>
-        <td class="none" align="center">No</td>
+        <td class="full" align="center">Clang 17</td>
       </tr>
     <tr>
       <td>Coroutines</td>