c++: Diagnose or avoid constexpr dtors in classes with virtual bases [PR114426]

I had another look at this P1 PR today.
You said in the "c++: fix in-charge parm in constexpr" mail back in December
(as well as in the r14-6507 commit message):
"Since a class with vbases can't have constexpr 'tors there isn't actually
a need for an in-charge parameter in a destructor" but the ICE is because
the destructor is marked implicitly constexpr.
https://eel.is/c++draft/dcl.constexpr#3.2 says that a destructor of a class
with virtual bases is not constexpr-suitable, but we were actually
implementing this just for constructors, so clearly my fault from the
https://wg21.link/P0784R7 implementation.  That paper clearly added that
sentence in there and removed similar sentence just from the constructor case.

So, the following patch makes sure the
  else if (CLASSTYPE_VBASECLASSES (DECL_CONTEXT (fun)))
    {
      ret = false;
      if (complain)
        error ("%q#T has virtual base classes", DECL_CONTEXT (fun));
    }
hunk is done no just for DECL_CONSTRUCTOR_P (fun), but also
DECL_DESTRUCTOR_P (fun) - in that case just for cxx_dialect >= cxx20,
as for cxx_dialect < cxx20 we already set ret = false; and diagnose
a different error, so no need to diagnose two.

2024-04-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114426
	* constexpr.cc (is_valid_constexpr_fn): Return false/diagnose with
	complain destructors in classes with virtual bases.

	* g++.dg/cpp2a/pr114426.C: New test.
	* g++.dg/cpp2a/constexpr-dtor16.C: New test.