Support indirect imports of namespaces. (#7122)

When a namespace that was imported from C++ is indirectly imported, find
the corresponding namespace in the current C++ AST and return that
instead. This namespace may have completely different contents than the
one we found before; that's fine. The current file's view of a namespace
depends on what it imported.

Assisted-by: Gemini via Antigravity

toolchain/check/import_ref.cpp

@@ -3699,11 +3699,24 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   auto name_id = GetLocalNameId(resolver, name_scope.name_id());
   namespace_decl.name_scope_id =
       resolver.local_name_scopes().Add(inst_id, name_id, parent_scope_id);
+  auto& local_scope =
+      resolver.local_name_scopes().Get(namespace_decl.name_scope_id);
   // Namespaces from this package are eagerly imported, so anything we load here
   // must be a closed import.
-  resolver.local_name_scopes()
-      .Get(namespace_decl.name_scope_id)
-      .set_is_closed_import(true);
+  local_scope.set_is_closed_import(true);
+
+  // If this was a C++ namespace, connect it to the corresponding C++
+  // declaration in this file.
+  if (name_scope.is_cpp_scope()) {
+    if (auto key = FindCorrespondingClangDeclKey(
+            resolver.local_context(), SemIR::LocId(inst_id),
+            resolver.import_ir(), name_scope.clang_decl_context_id())) {
+      auto clang_decl_id = resolver.local_context().clang_decls().Add(
+          {.key = *key, .inst_id = inst_id});
+      local_scope.set_clang_decl_context_id(clang_decl_id, true);
+    }
+  }
+
   auto namespace_const_id =
       ReplacePlaceholderImportedInst(resolver, inst_id, namespace_decl);
   return {.const_id = namespace_const_id};