[NFC] [C++] [Modules] Mark P1811 as implemented and add test (#146993)

This is already implemented. See godbolt:
https://godbolt.org/z/7x3arqj3G

Author: kish1n

clang/test/SemaCXX/P1811.cpp

@@ -0,0 +1,41 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+
+// RUN: %clang_cc1 -std=c++20 -verify -emit-module-interface %t/mod.cpp -o %t/mod.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify -fmodule-file=M=%t/mod.pcm %t/main1.cpp
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify -fmodule-file=M=%t/mod.pcm %t/main2.cpp
+
+//--- mod.cpp
+// expected-no-diagnostics
+module;
+#include "A.h"
+export module M;
+export A f() {return A{};}
+
+//--- A.h
+// expected-no-diagnostics
+#ifndef X
+#define X
+
+struct A{};
+
+#endif
+
+//--- main1.cpp
+// expected-no-diagnostics
+#include "A.h"
+import M;
+
+extern "C++" int main() {
+  A a;
+}
+
+//--- main2.cpp
+// expected-no-diagnostics
+import M;
+#include "A.h"
+
+extern "C++" int main() {
+  A a;
+}

clang/www/cxx_status.html

@@ -911,7 +911,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p1811r0">P1811R0</a></td>
-        <td class="none" align="center">No</td>
+        <td class="full" align="center">Clang 17</td>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p1703r1">P1703R1</a></td>