`-Wunused-private-field` not reported for `[[gnu::warn_unused]]` annotated type when empty constructors are present

```cpp
class [[gnu::warn_unused]] S1 {
public:
S1() {}
};

class [[gnu::warn_unused]] S2 {
public:
S2() = default;
};

class [[gnu::warn_unused]] S3 {
public:
S3();
};

class [[gnu::warn_unused]] S4 {
};

class C {
public:
C() {}

private:
const S1 s1; // no warning
const S2 s2; // warning
const S3 s3; // no warning
const S4 s4; // warning
};
```

```
:25:14: warning: private field 's2' is not used [-Wunused-private-field]
25 | const S2 s2; // warning
| ^
:27:14: warning: private field 's4' is not used [-Wunused-private-field]
27 | const S4 s4; // warning
| ^
```
https://godbolt.org/z/rjcEMzn96

If you change the `C` constructor to `= default` all are reported regardless of the other constructor.

Reproduce the warning behavior using the C++ example in the issue and the linked Compiler Explorer case. Compare the four annotated types with the user-provided and defaulted constructors, then compare with a defaulted constructor on C. Done means -Wunused-private-field reports all four fields consistently, with regression coverage for the reproducer.

Written by the indexing model from the issue text.