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/sem_ir/formatter.cpp

@@ -663,7 +663,8 @@ class FormatterImpl {
     const auto& scope = sem_ir_->name_scopes().Get(id);
 
     if (scope.entries().empty() && scope.extended_scopes().empty() &&
-        scope.import_ir_scopes().empty() && !scope.has_error()) {
+        scope.import_ir_scopes().empty() && !scope.is_cpp_scope() &&
+        !scope.has_error()) {
       // Name scope is empty.
       return;
     }
@@ -723,6 +724,11 @@ class FormatterImpl {
       out_ << "import " << label << "\n";
     }
 
+    if (scope.is_cpp_scope()) {
+      Indent();
+      out_ << "import Cpp//...\n";
+    }
+
     if (scope.has_error()) {
       Indent();
       out_ << "has_error\n";