c++: Consider addresses of heap artificial vars always non-NULL [PR98988, PR99031]

With -fno-delete-null-pointer-checks which is e.g. implied by
-fsanitize=undefined or default on some embedded targets, the middle-end
folder doesn't consider addresses of global VAR_DECLs to be non-NULL, as one
of them could have address 0.  Still, I think malloc/operator new (at least
the nonthrowing) relies on NULL returns meaning allocation failure rather
than success.  Furthermore, the artificial VAR_DECLs we create for
constexpr new never actually live in the address space of the program,
so we can pretend they will never be NULL too.

> I'm surprised that nonzero_address has such a limited set of things it will
> actually believe have non-zero addresses with
> -fno-delete-null-pointer-checks.  But it seems that we should be able to
> arrange to satisfy
>
> >   if (definition && !DECL_EXTERNAL (decl)
>
> since these "variables" are indeed defined within the current translation
> unit.

Doing that seems to work and as added benefit it fixes another PR that has
been filed recently.  I need to create the varpool node explicitly and call
a method that sets the definition member in there, but I can also unregister
those varpool nodes at the end of constexpr processing, as the processing
ensured they don't leak outside of the processing.

2021-02-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98988
	PR c++/99031
	* constexpr.c: Include cgraph.h.
	(cxx_eval_call_expression): Call varpool_node::finalize_decl on
	heap artificial vars.
	(cxx_eval_outermost_constant_expr): Remove varpool nodes for
	heap artificial vars.

	* g++.dg/cpp2a/constexpr-new16.C: New test.
	* g++.dg/cpp2a/constexpr-new17.C: New test.

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