[Clang] constant evaluator thinks that elements of an unnamed struct is not initialized

```c++
struct T {
  int i;
};

struct S {
  struct {
    T val;
  };

  constexpr S() : val() {}
  constexpr ~S() {}

  constexpr T get() { return val; }
};

constexpr bool func() {
  S s;
  return s.get().i == 0;
}

static_assert(func());
```

Here Clang says that `val` isn't initialized, even though it pretty clearly is.