When using a C++ struct as a parameter, map its type to a Carbon class type (#5538)

This doesn't support actually passing the value of the struct, which is
planned to be implemented using thunks.

`ClangDeclId` value is now `ClangDecl` which includes the mapped Carbon
instruction in addition to the Clang declaration. This allows finding
the Carbon instruction for a given Clang declaration, which is necessary
for mapping a Clang struct parameter type to the Carbon class without
doing name lookup. We don't take the instruction as part of the hash
key, as discussed in
[Discord](https://discord.com/channels/655572317891461132/768530752592805919/1380575881050718469).

To map the type, we also need to map namespaces. To avoid recursion for
inner namespaces, we use a vector.

Note that the first commit just changes the order of functions in the
file to make review easier.

C++ Interop Demo (that shows missing behavior):

```c++
// hello_world.h

struct S {
  S(const S&) { x = 1; }
  int x;
};

void hello_world(S s);
```

```c++
// hello_world.cpp

#include "hello_world.h"

#include <cstdio>

void hello_world2(S s) { printf("hello_world2: %d\n", s.x); }

void hello_world(S s) {
  printf("hello_world: %d\n", s.x);
  hello_world2(s);
}
```

```carbon
// main.carbon

library "Main";

import Cpp library "hello_world.h";

fn Run() -> i32 {
  var s : Cpp.S;
  Cpp.hello_world(s);
  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: -1108224096
hello_world2: 1
```

Part of #5533.

toolchain/lower/file_context.cpp

@@ -675,7 +675,7 @@ auto FileContext::BuildFunctionDecl(SemIR::FunctionId function_id,
     CARBON_CHECK(!specific_id.has_value(),
                  "Specific functions cannot have C++ definitions");
     HandleReferencedCppFunction(clang::dyn_cast<clang::FunctionDecl>(
-        sem_ir().clang_decls().Get(clang_decl_id)));
+        sem_ir().clang_decls().Get(clang_decl_id).decl));
     // TODO: Check that the signature and mangling generated by Clang and the
     // one we generated are the same.
   }