Add support for importing C++ namespaces (#5103)

This adds support for having different C++ `NameScope`s (and not just
the main `Cpp` scope), and we keep a pointer to `clang::DeclContext`
these scopes so we can look up C++ names in the right part of the AST.

C++ Interop Demo with a namespace:

```c++
// hello_world.h

namespace some_namespace {

void hello_world();

}  // namespace some_namespace
```

```c++
// hello_world.cpp

#include <cstdio>

namespace some_namespace {

void hello_world() { printf("Hello World!\n"); }

}  // namespace some_namespace
```

```carbon
// main.carbon

library "Main";

import Cpp library "hello_world.h";

fn Run() -> i32 {
  Cpp.some_namespace.hello_world();
  return 0;
}
```

```shell
$ clang -c hello_world.cpp
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link hello_world.o main.o --output=demo
$ ./demo
Hello World!
```

Closes #5102

toolchain/check/import_cpp.cpp

@@ -22,6 +22,7 @@
 #include "toolchain/check/type.h"
 #include "toolchain/diagnostics/diagnostic.h"
 #include "toolchain/diagnostics/format_providers.h"
+#include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/name_scope.h"
 
@@ -102,7 +103,7 @@ static auto AddNamespace(Context& context, PackageNameId cpp_package_id,
         {.node_id = import.node_id, .library_id = import.library_id});
   }
 
-  return AddImportNamespace(
+  return AddImportNamespaceToScope(
              context,
              GetSingletonType(context, SemIR::NamespaceType::SingletonInstId),
              SemIR::NameId::ForPackageName(cpp_package_id),
@@ -112,7 +113,7 @@ static auto AddNamespace(Context& context, PackageNameId cpp_package_id,
                return AddInst<SemIR::ImportCppDecl>(
                    context, imports.front().node_id, {});
              })
-      .name_scope_id;
+      .add_result.name_scope_id;
 }
 
 auto ImportCppFiles(Context& context, llvm::StringRef importing_file_path,
@@ -136,7 +137,8 @@ auto ImportCppFiles(Context& context, llvm::StringRef importing_file_path,
   auto name_scope_id = AddNamespace(context, package_id, imports);
   SemIR::NameScope& name_scope = context.name_scopes().Get(name_scope_id);
   name_scope.set_is_closed_import(true);
-  name_scope.set_is_cpp_scope(true);
+  name_scope.set_cpp_decl_context(
+      generated_ast->getASTContext().getTranslationUnitDecl());
 
   context.sem_ir().set_cpp_ast(generated_ast.get());
 
@@ -147,10 +149,10 @@ auto ImportCppFiles(Context& context, llvm::StringRef importing_file_path,
   return std::move(generated_ast);
 }
 
-// Look ups the given name in the Clang AST. Returns the lookup result if lookup
-// was successful.
+// Look ups the given name in the Clang AST in a specific scope. Returns the
+// lookup result if lookup was successful.
 static auto ClangLookup(Context& context, SemIR::LocId loc_id,
-                        SemIR::NameId name_id)
+                        SemIR::NameScopeId scope_id, SemIR::NameId name_id)
     -> std::optional<clang::LookupResult> {
   std::optional<llvm::StringRef> name =
       context.names().GetAsStringIfIdentifier(name_id);
@@ -172,7 +174,7 @@ static auto ClangLookup(Context& context, SemIR::LocId loc_id,
       clang::Sema::LookupNameKind::LookupOrdinaryName);
 
   bool found = sema.LookupQualifiedName(
-      lookup, ast->getASTContext().getTranslationUnitDecl());
+      lookup, context.name_scopes().Get(scope_id).cpp_decl_context());
 
   if (lookup.isClassLookup()) {
     // TODO: To support class lookup, also return the AccessKind for storage.
@@ -250,17 +252,37 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
   return decl_id;
 }
 
+// Imports a namespace declaration from Clang to Carbon. If successful, returns
+// the new Carbon namespace declaration `InstId`.
+static auto ImportNamespaceDecl(Context& context,
+                                SemIR::NameScopeId parent_scope_id,
+                                SemIR::NameId name_id,
+                                clang::NamespaceDecl* clang_decl)
+    -> SemIR::InstId {
+  auto result = AddImportNamespace(
+      context, GetSingletonType(context, SemIR::NamespaceType::SingletonInstId),
+      name_id, parent_scope_id, /*import_id=*/SemIR::InstId::None);
+  context.name_scopes()
+      .Get(result.name_scope_id)
+      .set_cpp_decl_context(clang_decl);
+  return result.inst_id;
+}
+
 // Imports a declaration from Clang to Carbon. If successful, returns the
 // instruction for the new Carbon declaration.
 static auto ImportNameDecl(Context& context, SemIR::LocId loc_id,
                            SemIR::NameScopeId scope_id, SemIR::NameId name_id,
-                           const clang::NamedDecl* clang_decl)
-    -> SemIR::InstId {
+                           clang::NamedDecl* clang_decl) -> SemIR::InstId {
   if (const auto* clang_function_decl =
           clang::dyn_cast<clang::FunctionDecl>(clang_decl)) {
     return ImportFunctionDecl(context, loc_id, scope_id, name_id,
                               clang_function_decl);
   }
+  if (auto* clang_namespace_decl =
+          clang::dyn_cast<clang::NamespaceDecl>(clang_decl)) {
+    return ImportNamespaceDecl(context, scope_id, name_id,
+                               clang_namespace_decl);
+  }
 
   context.TODO(loc_id, llvm::formatv("Unsupported: Declaration type {0}",
                                      clang_decl->getDeclKindName())
@@ -271,7 +293,7 @@ static auto ImportNameDecl(Context& context, SemIR::LocId loc_id,
 auto ImportNameFromCpp(Context& context, SemIR::LocId loc_id,
                        SemIR::NameScopeId scope_id, SemIR::NameId name_id)
     -> SemIR::InstId {
-  auto lookup = ClangLookup(context, loc_id, name_id);
+  auto lookup = ClangLookup(context, loc_id, scope_id, name_id);
   if (!lookup) {
     return SemIR::InstId::None;
   }