[NFC] [C++] [Modules] Mark P1979 as implemented and add test (#146841)

Proposal
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1979r0.html
appears to already be implemented as of Clang17. See:
https://godbolt.org/z/h59Mvzq6P

Author: kish1n

clang/test/SemaCXX/P1979.cpp

@@ -0,0 +1,38 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+
+// RUN: %clang_cc1 -std=c++20 -verify -emit-module-interface %t/A.cpp -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 -verify -emit-module-interface %t/myV.cpp -o %t/myV.pcm
+// RUN: %clang_cc1 -std=c++20 -verify -emit-module-interface -fmodule-file=V=%t/myV.pcm %t/partition.cpp -o %t/partition.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify -fmodule-file=V=%t/myV.pcm -fmodule-file=A=%t/A.pcm -fmodule-file=A:partition=%t/partition.pcm %t/interface.cpp
+
+//--- A.cpp
+// expected-no-diagnostics
+export module A;
+
+//--- myV.cpp
+// expected-no-diagnostics
+export module V;
+
+export struct myV{};
+
+//--- uses_vector.h
+// expected-no-diagnostics
+#ifndef x
+#define x
+
+import V;
+#endif
+
+//--- partition.cpp
+// expected-no-diagnostics
+module;
+#include "uses_vector.h" // textually expands to import V;
+module A:partition;
+
+//--- interface.cpp
+module A;
+import :partition;
+myV V; // expected-error {{declaration of 'myV' must be imported from module 'V' before it is required}}
+       // expected-note@myV.cpp:4 {{declaration here is not visible}}

clang/www/cxx_status.html

@@ -923,7 +923,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr> <!-- from Belfast -->
         <td><a href="https://wg21.link/p1979r0">P1979R0</a></td>
-        <td class="none" align="center">No</td>
+        <td class="full" align="center">Clang 17</td>
       </tr>
       <tr> <!-- from Prague -->
         <td><a href="https://wg21.link/p1779r3">P1779R3</a></td>