Add support for importing access from C++ to Carbon (#5858)

Better access control with inheritance should come with better
inheritance support (actually importing inheritance).

C++ Interop Demo:

```c++
// hello_world.h

class HelloWorld {
 public:
  static auto Pub() -> void;

 protected:
  static auto Pro() -> void;

 private:
  static auto Pri() -> void;
};
```

```c++
// hello_world.cpp

#include "hello_world.h"

#include <cstdio>

auto HelloWorld::Pub() -> void { printf("Public!\n"); }
auto HelloWorld::Pro() -> void { printf("Protected!\n"); }
auto HelloWorld::Pri() -> void { printf("Private!\n"); }
```

```carbon
// main.carbon

library "Main";

import Cpp library "hello_world.h";

fn Run() -> i32 {
  Cpp.HelloWorld.Pub();
  Cpp.HelloWorld.Pro();
  Cpp.HelloWorld.Pri();
  return 0;
}
```

```shell
$ clang -c hello_world.cpp
$ bazel-bin/toolchain/carbon compile main.carbon
main.carbon:9:3: error: cannot access protected member `Pro` of type `Cpp.HelloWorld`
  Cpp.HelloWorld.Pro();
  ^~~~~~~~~~~~~~~~~~
main.carbon: note: declared here

main.carbon:10:3: error: cannot access private member `Pri` of type `Cpp.HelloWorld`
  Cpp.HelloWorld.Pri();
  ^~~~~~~~~~~~~~~~~~
main.carbon: note: declared here
```

Before this change (no access checks):
```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link hello_world.o main.o --output=demo
$ ./demo
Public!
Protected!
Private!
```

Part of #5859.

toolchain/check/testdata/interop/cpp/function/class.carbon

@@ -266,6 +266,7 @@ fn F() {
 // --- definition_with_static_method.h
 
 class C {
+ public:
   static void bar();
 };