Clang accepts a constructor template whose default argument initializes int from nullptr

Clang 22 accepts the following C++20 program, while Clang GCC 15.2.0 rejects it:

```c++
struct C {
C() {}

template
C(C&, T = nullptr) {}
};

C c;

struct A {
explicit operator C&() const { return c; }
};

int main() {
A a;
C x(a);
}
```

Clang: clang++-22 22.0.0
-std=c++20: accepts
-std=c++20 -pedantic-errors: accepts

GCC: gcc15 15.2.0
-std=c++20: rejects
-std=c++20 -pedantic-errors: rejects

GCC diagnostic:

```
error: could not convert 'nullptr' from 'std::nullptr_t' to 'int'
note: when instantiating default argument for call to 'C::C(C&, T) [with T = int]'
```