C++ interop: Support importing operators defined in namespaces (#6024)

C++ Interop Demo:

```c++
// 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
```

```c++
// my_number.cpp

#include "my_number.h"

namespace MyNamespace {

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

}  // namespace MyNamespace
```

```carbon
// 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:

```shell
$ 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 https://github.com/carbon-language/carbon-lang/issues/5995.

toolchain/check/import_cpp.cpp

@@ -2211,7 +2211,8 @@ static auto GetClangOperatorKind(Context& context, SemIR::LocId loc_id,
   return std::nullopt;
 }
 
-auto ImportOperatorFromCpp(Context& context, SemIR::LocId loc_id, Operator op)
+auto ImportOperatorFromCpp(Context& context, SemIR::LocId loc_id,
+                           SemIR::NameScopeId scope_id, Operator op)
     -> SemIR::ScopeLookupResult {
   Diagnostics::AnnotationScope annotate_diagnostics(
       &context.emitter(), [&](auto& builder) {
@@ -2231,7 +2232,7 @@ auto ImportOperatorFromCpp(Context& context, SemIR::LocId loc_id, Operator op)
   // into C++ types. See
   // https://github.com/carbon-language/carbon-lang/pull/5996/files/5d01fa69511b76f87efbc0387f5e40abcf4c911a#r2316950123
   auto decl_and_access = ClangLookupDeclarationName(
-      context, loc_id, SemIR::NameScopeId::None,
+      context, loc_id, scope_id,
       context.ast_context().DeclarationNames.getCXXOperatorName(*op_kind));
 
   if (!decl_and_access) {