[NFC] [C++] [Modules] Mark P2615 as implemented and add test (#147135)

This is already implemented. See godbolt:
https://godbolt.org/z/1ra4Ka8od

Author: kish1n

clang/test/SemaCXX/P2615.cpp

@@ -0,0 +1,11 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+
+// RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %t/A.cpp
+
+//--- A.cpp
+// expected-no-diagnostics
+export module A;
+export namespace N {int x = 42;}
+export using namespace N;

clang/www/cxx_status.html

@@ -943,7 +943,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr> <!-- from Kona 2022 -->
         <td><a href="https://wg21.link/P2615R1">P2615R1</a> (<a href="#dr">DR</a>)</td>
-        <td class="none" align="center">No</td>
+        <td class="full" align="center">Clang 17</td>
       </tr>
       <tr> <!-- from Issaquah 2023 -->
         <td><a href="https://wg21.link/P2788R0">P2788R0</a> (<a href="#dr">DR</a>)</td>