[Clang19][C++20] is_default_constructible assert fails conditionally on whether a member variable of that type is present in a class or not

With clang 19.1.0 and c++20 the trait is_default_constructible results in different results for a pair containing a fundamental type and an aggregate with DMI (direct member initialisation) in a class if a member variable of such a pair type exists in the class or not.

Specifically the following fails to compile with the static_assert failing (https://godbolt.org/z/sGfcrnczx)
```cpp
#include
#include

class C {
public:
struct S {
int i = 0;
};
using test_type = std::pair;
test_type p;
C() {}
};

int main() {
C{};
static_assert(std::is_default_constructible::value, "failed");
return 0;
}
```

However the following code compiles just fine with the static_assert passing (https://godbolt.org/z/15f9b5eGd)

```cpp
#include
#include

class C {
public:
struct S {
int i = 0;
};
using test_type = std::pair;
// test_type p;
C() {}
};

int main() {
C{};
static_assert(std::is_default_constructible::value, "failed");
return 0;
}
```

Reproduce the two C++20 examples with Clang 19.1.0 using the linked Godbolt cases, comparing the results with and without the member variable. Trace the implementation of std::is_default_constructible and the relevant Clang diagnostic or type-trait handling; done means the trait gives the same expected result for both class definitions and the reproductions compile.

Written by the indexing model from the issue text.