Fix 22951 - Emit non-extern (C++) destructors as private members

[MoonlightSentinel]

Destructors not marked as extern (C++) aren't accessible from C++
due to the D name mangling. The header generator used to skip extern(D)
destructors, allowing C++ code that violated RAII guarantees.

Declaring the constructors as private members ensures that any
instance that would need to be destroyed on the C++ side causes a
compiler error (rather than a linker error due to missmatched mangling).

Renamed the helper function checkVirtualFunction because it now also
handles other types of functions.

[dlang-bot]

Thanks for your pull request and interest in making D better, @MoonlightSentinel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22951	normal	Dtor missing from generated C++ header

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#13914"