c++: Fix up constexpr evaluation of deleting dtors [PR100495]

c++: Fix up constexpr evaluation of deleting dtors [PR100495]

We do not save bodies of constexpr clones and instead evaluate the bodies
of the constexpr functions they were cloned from.
I believe that is just fine for constructors because complete vs. base
ctors differ only in classes that have virtual bases and such constructors
aren't constexpr, similarly complete/base destructors.
But as the testcase below shows, for deleting destructors it is not fine,
deleting dtors while marked as clones in fact are just artificial functions
with synthetized body which calls the user destructor and deallocation.

So, either we'd need to evaluate the destructor and afterwards synthetize
and evaluate the deallocation, or we can just save and use the deleting
dtors bodies.  The latter seems much easier to me.

2021-09-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/100495
	* constexpr.c (maybe_save_constexpr_fundef): Save body even for
	constexpr deleting dtors.
	(cxx_eval_call_expression): Don't use DECL_CLONED_FUNCTION for
	deleting dtors.

	* g++.dg/cpp2a/constexpr-new21.C: New test.