C++ interop: Support importing operators defined in namespaces

[bricknerb]

C++ Interop Demo:

// my_number.h

namespace MyNamespace {

class MyNumber {
 public:
  explicit MyNumber(int value) : value_(value) {}
  auto value() const -> int { return value_; }

 private:
  int value_;
};

auto operator+(MyNumber lhs, MyNumber rhs) -> MyNumber;

}  // namespace MyNamespace

// my_number.cpp

#include "my_number.h"

namespace MyNamespace {

auto operator+(MyNumber lhs, MyNumber rhs) -> MyNumber {
  return MyNumber(lhs.value() + rhs.value());
}

}  // namespace MyNamespace

// main.carbon

library "Main";

import Core library "io";
import Cpp library "my_number.h";

fn Run() -> i32 {
  let n1: Cpp.MyNamespace.MyNumber = Cpp.MyNamespace.MyNumber.MyNumber(5);
  Core.Print(n1.value());
  let n2: Cpp.MyNamespace.MyNumber = Cpp.MyNamespace.MyNumber.MyNumber(7);
  Core.Print(n2.value());
  let n3: Cpp.MyNamespace.MyNumber = n1 + n2;
  Core.Print(n3.value());
  return 0;
}

Before this change:

$ bazel-bin/toolchain/carbon compile main.carbon
main.carbon:13:38: error: cannot access member of interface `Core.AddWith(Cpp.MyNamespace.MyNumber)` in type `Cpp.MyNamespace.MyNumber` that does not implement that interface
  let n3: Cpp.MyNamespace.MyNumber = n1 + n2;
                                     ^~~~~~~

With this change:

$ clang -c my_number.cpp
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link my_number.o main.o --output=demo
$ ./demo
5
7
12

Part of #5995.

[ivanaivanovska]

LGTM.
A nit suggestion: rewrite the demo example to show the Before state at the top, or make it better visible. I got confused as I thought it was the result of the current compilation.

[zygoloid]

Looks good to me too, but I think we should at least add a TODO to use ADL once we can.

[zygoloid]

Can we call Sema::ArgumentDependentLookup here instead of reimplementing something similar? I think we'd need #5891 to land first, so that we have expressions corresponding to the arguments at the point where we do the lookup.

If you'd prefer to do it this way for the time being, I think that's fine, but please add a TODO to replace this with a call to ArgumentDependentLookup when we can.

[bricknerb]

Right, waiting for #5891.
Added TODOs for now (similar to the one in import_cpp.cpp.

[bricknerb]

LGTM. A nit suggestion: rewrite the demo example to show the Before state at the top, or make it better visible. I got confused as I thought it was the result of the current compilation.

Done.