re PR c++/91369 (Implement P0784R7: constexpr new)

re PR c++/91369 (Implement P0784R7: constexpr new)

	PR c++/91369
	* constexpr.c (struct constexpr_global_ctx): Add heap_alloc_count
	member, initialize it to zero in ctor.
	(cxx_eval_call_expression): Bump heap_dealloc_count when deleting
	a heap object.  Don't cache calls to functions which allocate some
	heap objects and don't deallocate them or deallocate some heap
	objects they didn't allocate.

	* g++.dg/cpp1y/constexpr-new.C: Expect an error explaining why
	static_assert failed for C++2a.
	* g++.dg/cpp2a/constexpr-new9.C: New test.

From-SVN: r279943