[C++20][modules] Fix std::initializer_list recognition if it's exported out of a module (#118537)

If the std::initializer_list is exported out of module, its
`DeclContext` is not a namespace as `Sema::isStdInitializerList`
expects, but an `Decl::Kind::Export` and only its parent is a namespace.
So this commit makes `Sema::isStdInitializerList` account for that.

I'm really new to clang so I'm not 100% sure that was the issue, it
seems so and it fixes compilation. Also I probably need to add tests but
I'd like someone to approve the idea first.

Fixes #118218

Author: jijjijj

clang/docs/ReleaseNotes.rst

@@ -810,6 +810,8 @@ Bug Fixes to C++ Support
 - Fixed an incorrect lambda scope of generic lambdas that caused Clang to crash when computing potential lambda
   captures at the end of a full expression. (#GH115931)
 - Clang no longer rejects deleting a pointer of incomplete enumeration type. (#GH99278)
+- Fixed recognition of ``std::initializer_list`` when it's surrounded with ``extern "C++"`` and exported
+  out of a module (which is the case e.g. in MSVC's implementation of ``std`` module). (#GH118218)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDeclCXX.cpp

@@ -11931,7 +11931,7 @@ bool Sema::isStdInitializerList(QualType Ty, QualType *Element) {
     if (TemplateClass->getIdentifier() !=
             &PP.getIdentifierTable().get("initializer_list") ||
         !getStdNamespace()->InEnclosingNamespaceSetOf(
-            TemplateClass->getDeclContext()))
+            TemplateClass->getNonTransparentDeclContext()))
       return false;
     // This is a template called std::initializer_list, but is it the right
     // template?

clang/test/Modules/initializer-list-recognition-through-export-and-linkage-issue-118218.cpp

@@ -0,0 +1,39 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/std.cppm -emit-module-interface -o %t/std.pcm
+// RUN: %clang_cc1 -std=c++20 %t/mod.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/mod.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %t/main.cpp
+
+//--- std.cppm
+export module std;
+
+extern "C++" {
+  namespace std {
+  export template <class E>
+  class initializer_list {
+    const E* _1;
+    const E* _2;
+  };
+  }
+}
+
+//--- mod.cppm
+export module mod;
+
+import std;
+
+export struct A {
+  void func(std::initializer_list<int>) {}
+};
+
+//--- main.cpp
+// expected-no-diagnostics
+import std;
+import mod;
+
+int main() {
+  A{}.func({1,1});
+  return 0;
+}