[C++20] [Modules] Namespace Declaration shouldn't have module linkage

[ChuanqiXu]

Summary

According to https://eel.is/c++draft/basic.namespace.general#2, a namespace declaration shouldn't have a module linkage.

Further more, without this patch, the compiler would crash for the test in assertion enabled build due to inconsistent linkage for redeclaration for namespaces.

Test Plan: check-all

clang/lib/AST/Decl.cpp

static LinkageInfo getInternalLinkageFor(const NamedDecl *D) {
   return LinkageInfo::internal();
 }
 
 static LinkageInfo getExternalLinkageFor(const NamedDecl *D) {
   // C++ Modules TS [basic.link]/6.8:
   //   - A name declared at namespace scope that does not have internal linkage
   //     by the previous rules and that is introduced by a non-exported
   //     declaration has module linkage.
-  if (isInModulePurview(D) && !isExportedFromModuleInterfaceUnit(
-                                  cast<NamedDecl>(D->getCanonicalDecl())))
+  //
+  // [basic.namespace.general]/p2
+  //   A namespace is never attached to a named module and never has a name with
+  //   module linkage.
+  if (isInModulePurview(D) &&
+      !isExportedFromModuleInterfaceUnit(
+          cast<NamedDecl>(D->getCanonicalDecl())) &&
+      !isa<NamespaceDecl>(D))
     return LinkageInfo(ModuleLinkage, DefaultVisibility, false);
 
   return LinkageInfo::external();
 }
 
 static StorageClass getStorageClass(const Decl *D) {
   if (auto *TD = dyn_cast<TemplateDecl>(D))
     D = TD->getTemplatedDecl();

clang/test/CXX/basic/basic.namespace/basic.namespace.general/Inputs/p2.h

+#ifndef P2_H
+#define P2_H
+namespace foo {
+namespace bar {
+}
+} // namespace foo
+#endif

clang/test/CXX/basic/basic.namespace/basic.namespace.general/Inputs/p2.cppm

+module;
+#include "p2.h"
+export module Y;
+export namespace foo {
+// We need to export something at least
+void print();
+}

clang/test/CXX/basic/basic.namespace/basic.namespace.general/p2.cppm

+// Check that the compiler wouldn't crash due to inconsistent namesapce linkage
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/p2.cppm --precompile -o %t/Y.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t -I%S/Inputs %s -c -Xclang -verify
+// expected-no-diagnostics
+export module X;
+import Y;
+
+export namespace foo {
+namespace bar{
+    void baz();
+}
+}