When using a C++ struct as a parameter, map it to a Carbon type

[bricknerb]

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

I plan to add more tests in a separate PR.

Based on #5468.

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

// hello_world.h

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

void hello_world(S s);

// 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);
}

// main.carbon

library "Main";

import Cpp library "hello_world.h";

fn Run() -> i32 {
  var s : Cpp.S;
  Cpp.hello_world(s);
  return 0;
}

$ 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.

[geoffromer]

If I understand correctly, you added this to prevent a crash that looks like this:

CHECK failure at toolchain/check/pattern_match.cpp:379: static_cast<size_t>(param_pattern.index.index) == results_.size(): Parameters out of order; expecting 0 but got -1

If so, I think that crash indicates violation of an invariant that CalleePatternMatch is supposed to establish, but the import logic isn't calling CalleePatternMatch. Instead MakeParamPatternsBlockId is trying to emulate it, and there are some bugs in the emulation (see also here). I'd strongly encourage you to use CalleePatternMatch whenever you're creating an EntityWithParams. If there are major obstacles to doing that, let me know; I'm happy to help figure out a solution.

[bricknerb]

Thanks!
It seems like CalleePatternMatch() would be called as part of resolving #5449, so I'll hold off this change until we manage to do that. I know @ivanaivanovska has been working on this as her top priority.

[ivanaivanovska]

remove todo_?

[bricknerb]

Done.

[ivanaivanovska]

Can we add a test for the scope? (e.g. the name not found in the scope)