[C++20] [Modules] Use current named module to do module local lookup

See the attached test for the motiviation.

Previously we dependent on the module ownership of the decl context to
perform module local lookup. But if the lookup is unqualified, we may
perform the lookup with canonical decl, which belongs to the incorrect
named module

Author: ChuanqiXu9

clang/lib/Serialization/ASTReader.cpp

@@ -8564,14 +8564,22 @@ bool ASTReader::FindExternalVisibleDeclsByName(const DeclContext *DC,
     Find(It->second.Table, Name);
   }
 
-  if (auto *NamedModule =
-          OriginalDC ? cast<Decl>(OriginalDC)->getTopLevelOwningNamedModule()
-                     : nullptr) {
+  auto FindModuleLocalLookup = [&, this](Module *NamedModule) {
     if (auto It = ModuleLocalLookups.find(DC); It != ModuleLocalLookups.end()) {
       ++NumModuleLocalVisibleDeclContexts;
       Find(It->second.Table, std::make_pair(Name, NamedModule));
     }
-  }
+  };
+  if (auto *NamedModule =
+          OriginalDC ? cast<Decl>(OriginalDC)->getTopLevelOwningNamedModule()
+                     : nullptr)
+    FindModuleLocalLookup(NamedModule);
+  // See clang/test/Modules/ModulesLocalNamespace.cppm for the motiviation case.
+  // We're going to find a decl but the decl context of the lookup is
+  // unspecified. In this case, the OriginalDC may be the decl context in other
+  // module.
+  if (ContextObj && ContextObj->getCurrentNamedModule())
+    FindModuleLocalLookup(ContextObj->getCurrentNamedModule());
 
   if (auto It = TULocalLookups.find(DC); It != TULocalLookups.end()) {
     ++NumTULocalVisibleDeclContexts;

clang/test/Modules/ModulesLocalNamespace.cppm

@@ -0,0 +1,47 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m-a.cppm -emit-module-interface -o %t/m-a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -emit-module-interface -o %t/m-b.pcm \
+// RUN:     -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/b.cpp -fsyntax-only -verify -fprebuilt-module-path=%t
+
+//--- a.cppm
+export module a;
+namespace aa {
+
+}
+
+//--- m-a.cppm
+export module m:a;
+namespace aa {
+struct A {};
+}
+
+//--- b.cppm
+module m:b;
+import :a;
+
+namespace bb {
+struct B {
+    void func(aa::A);
+};
+}
+
+//--- b.cpp
+// expected-no-diagnostics
+module m:b.impl;
+import a;
+import :b;
+
+namespace bb {
+using namespace aa;
+
+void B::func(A) {} 
+
+}
+
+
+