[CI] Fix clang-18 C++23 build of cppfront

[bluetarpmedia]

Some constructors and destructors have to be defaulted/written out-of-line in order to compile successfully with clang 18 in C++23 mode, due to forward declaring types and using them with unique_ptr.

Fixes this build error.

Similar issue as solved by this previous PR: #1088

EDIT: This only fixes the CI build of cppfront; I'll create a separate PR to update the regression tests.

[hsutter]

Thanks! But this is only needed for the destructor, right? Not the constructor?

[bluetarpmedia]

It's needed for both in this case.

Here's a repro showing the ctor issue:

#include <memory>

struct MyType;

struct Owner
{
    std::unique_ptr<MyType> mine_;

    Owner() {}
};

struct MyType
{
    int value = 0;
};

int main()
{
    Owner o;
}

Relevant hint from the error:

<source>:9:5: note: in instantiation of member function 'std::unique_ptr<MyType>::~unique_ptr' requested here
    9 |     Owner() {}

I think it needs to be able to destruct the unique_ptr in case the ctor throws?

Fixed version:

#include <memory>

struct MyType;

struct Owner
{
    std::unique_ptr<MyType> mine_;

    Owner();
};

struct MyType
{
    int value = 0;
};

Owner::Owner() {}

int main()
{
    Owner o;
}

[hsutter]

I think it needs to be able to destruct the unique_ptr in case the ctor throws?

That reasoning makes sense to me. Then I wondered, 'why do other types in parse.h only have to put the dtor out of line for this?' I looked, and I think the answer is because those types don't have a user-declared constructor... bingo.

So I think this makes sense. Thanks!