C++ interop: Support unary operators (#6020)

Newly supported: `-`.
Partially supported due to lack of reference support: `++` (prefix),
`--` (prefix).
Not supported due to lack of Carbon support to call them correctly: `+`,
`++` (postfix), `--` (postfix), `~`, `!`, `&`, `*`, `->`.

Also (for consistency):
* Add the operator declarations to unsupported binary operators tests.
* Logical operators and the unary `operator&` (address of) are expected
to be called by explicitly calling `operatorX`.

C++ Interop Demo:

```c++
// my_number.h

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

 private:
  int value_;
};

auto operator++(MyNumber operand) -> MyNumber;
auto operator--(MyNumber operand) -> MyNumber;
auto operator-(MyNumber operand) -> MyNumber;
```

```c++
// my_number.cpp

#include "my_number.h"

auto operator++(MyNumber operand) -> MyNumber {
  return MyNumber(operand.value() + 1);
}

auto operator--(MyNumber operand) -> MyNumber {
  return MyNumber(operand.value() - 1);
}

auto operator-(MyNumber operand) -> MyNumber {
  return MyNumber(-operand.value());
}
```

```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(14);
  Core.Print(num.value());
  ++num;
  Core.Print(num.value());
  --num;
  Core.Print(num.value());
  num = -num;
  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
14
14
14
-14
```

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

toolchain/check/import_cpp.cpp

@@ -2076,6 +2076,31 @@ static auto GetClangOperatorKind(Context& context, SemIR::LocId loc_id,
                                  llvm::StringLiteral interface_name,
                                  llvm::StringLiteral op_name)
     -> std::optional<clang::OverloadedOperatorKind> {
+  // Unary operators.
+  if (interface_name == "Destroy" || interface_name == "As" ||
+      interface_name == "ImplicitAs") {
+    // TODO: Support destructors and conversions.
+    return std::nullopt;
+  }
+
+  // Increment and Decrement.
+  if (interface_name == "Inc") {
+    CARBON_CHECK(op_name == "Op");
+    return clang::OO_PlusPlus;
+  }
+  if (interface_name == "Dec") {
+    CARBON_CHECK(op_name == "Op");
+    return clang::OO_MinusMinus;
+  }
+
+  // Arithmetic.
+  if (interface_name == "Negate") {
+    CARBON_CHECK(op_name == "Op");
+    return clang::OO_Minus;
+  }
+
+  // Binary operators.
+
   // Arithmetic Operators.
   if (interface_name == "AddWith") {
     CARBON_CHECK(op_name == "Op");