[NFC] [C++] [Modules] Mark P2115 as implemented and add test (llvm#147489)

This is already implemented. Proposal:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2115r0.html

Author: kish1n

clang/test/SemaCXX/P2115.cpp

@@ -0,0 +1,21 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+
+// RUN: %clang -std=c++20 -fmodule-header %t/A.h -o %t/A.pcm
+// RUN: %clang -std=c++20 -fmodule-header %t/B.h -o %t/B.pcm
+// RUN: %clang -std=c++20 -fsyntax-only -fmodule-file=%t/A.pcm -fmodule-file=%t/B.pcm %t/main.cpp
+
+//--- A.h
+// expected-no-diagnostics
+enum { A = 0 };
+
+//--- B.h
+// expected-no-diagnostics
+enum { B = 1 };
+
+//--- main.cpp
+// expected-no-diagnostics
+import "A.h";
+import "B.h";
+int main() {}

clang/www/cxx_status.html

@@ -935,7 +935,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p2115r0">P2115R0</a></td>
-        <td class="partial" align="center">Partial</td>
+        <td class="full" align="center">Yes</td>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p1815r2">P1815R2</a></td>