c++: Fix ICE during constexpr virtual call evaluation [PR93633]

c++: Fix ICE during constexpr virtual call evaluation [PR93633]

The first (valid) testcase ICEs because for
  A *a = new B ();
  a->foo (); // virtual method call
we actually see &heap  and the "heap " objects don't have the class or
whatever else type was used in new expression, but an array type containing
one (or more of those for array new) and so when using TYPE_BINFO (objtype)
on it we ICE.
This patch handles this special case, and otherwise punts (as shown e.g. in
the second testcase, where because the heap object is already deleted,
we don't really want to allow it to be used.

2020-02-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93633
	* constexpr.c (cxx_eval_constant_expression): If obj is heap var with
	ARRAY_TYPE, use the element type.  Punt if objtype after that is not
	a class type.

	* g++.dg/cpp2a/constexpr-new11.C: New test.
	* g++.dg/cpp2a/constexpr-new12.C: New test.
	* g++.dg/cpp2a/constexpr-new13.C: New test.