C++ Interop: Add support for <<= and >>= (#6325)

C++ Interop Demo:

```c++
// my_number.h

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

 private:
  int value_;
};

auto operator<<=(MyNumber& lhs, int rhs) -> MyNumber&;
auto operator>>=(MyNumber& lhs, int rhs) -> MyNumber&;
```

```c++
// my_number.cpp

#include "my_number.h"

auto operator<<=(MyNumber& lhs, int rhs) -> MyNumber& {
  lhs.set_value(lhs.value() << rhs);
  return lhs;
}
auto operator>>=(MyNumber& lhs, int rhs) -> MyNumber& {
  lhs.set_value(lhs.value() >> rhs);
  return lhs;
}
```

```carbon
// main.carbon

library "Main";

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

fn Run() -> i32 {
  var num: Cpp.MyNumber = Cpp.MyNumber.MyNumber(3);
  Core.Print(num.value());
  num <<= 2;
  Core.Print(num.value());
  num >>= 1;
  Core.Print(num.value());
  return 0;
}
```

```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
3
12
6
```

Part of https://github.com/carbon-language/carbon-lang/issues/5995.

toolchain/check/cpp/operators.cpp

@@ -125,9 +125,14 @@ static auto GetClangOperatorKind(Context& context, SemIR::LocId loc_id,
     CARBON_CHECK(op_name == "Op");
     return clang::OO_CaretEqual;
   }
-  // TODO: Add support for `LeftShiftAssignWith` (`OO_LessLessEqual`) and
-  // `RightShiftAssignWith` (`OO_GreaterGreaterEqual`) when references are
-  // supported.
+  if (interface_name == "LeftShiftAssignWith") {
+    CARBON_CHECK(op_name == "Op");
+    return clang::OO_LessLessEqual;
+  }
+  if (interface_name == "RightShiftAssignWith") {
+    CARBON_CHECK(op_name == "Op");
+    return clang::OO_GreaterGreaterEqual;
+  }
 
   // Relational Operators.
   if (interface_name == "EqWith") {