[C++20] [Modules] Make module local decls visible to language linkage in the same module

Close https://github.com/llvm/llvm-project/issues/123343

See the issue and the comments in the patch for details.

clang/lib/Sema/SemaLookup.cpp

@@ -1624,6 +1624,11 @@ bool Sema::isUsableModule(const Module *M) {
   if (!Current)
     return false;
 
+  // For implicit global module, the decls in the same modules with the parent
+  // module should be visible to the decls in the implicit global module.
+  if (Current->isImplicitGlobalModule())
+    Current = Current->getTopLevelModule();
+
   // If M is the module we're parsing or M and the current module unit lives in
   // the same module, M should be usable.
   //

clang/test/Modules/module-local-visibility-in-language-linkage.cppm

@@ -0,0 +1,16 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/m.a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m.b.cppm -fmodule-file=m:a=%t/a.pcm -fsyntax-only -verify
+
+//--- m.a.cppm
+export module m:a;
+int a;
+
+//--- m.b.cppm
+// expected-no-diagnostics
+module m:b;
+import :a;
+extern "C++" int get_a() { return a; }