Add support for nullptr literals in macros (#6426)

Adding support for macros that evaluate to nullptr literal.

Demo:

```c++
// macros.h
void foo(int a[2]);
#define MyNullPtr nullptr
```
```c++
// macros.cpp
void foo(int a[2]) {
  if (!a) {
    printf("array a is nullptr\n");
    return;
  }
  printf("a[0] = %d \n", a[0]);
}
```

```c++
// main.carbon

library "Main";

import Cpp library "macros.h";

fn Run() -> i32 {
  Cpp.foo(Cpp.MyNullPtr);
  return 0;
}
```

```
$ clang -c macros.cpp;
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link macros.o main.o \--output=demo_carbon
$ ./demo_carbon
array a is nullptr
```

Part of #6303

toolchain/check/cpp/macros.cpp

@@ -62,7 +62,8 @@ auto TryEvaluateMacroToConstant(Context& context, SemIR::LocId loc_id,
   }
 
   if (isa<clang::StringLiteral>(result_expr) ||
-      isa<clang::CharacterLiteral>(result_expr)) {
+      isa<clang::CharacterLiteral>(result_expr) ||
+      isa<clang::CXXNullPtrLiteralExpr>(result_expr)) {
     return result_expr;
   }