cp: Explicitly null-derive some pointers

cp: Explicitly null-derive some pointers

The C++ frontend sometimes constructs pointers from constant integers by
directly converting them. This patch null-derives these instead in a
couple of places to avoid ICEs.

gcc/cp/ChangeLog:

	* class.c (build_rtti_vtbl_entries): We can't simply convert
	integers into capability pointers: null-derive these instead
	(e.g. when putting an offset into a vtable).
	* method.c (build_stub_object): When building our dummy
	reference, null-derive a capability instead of just trying to
	convert integer_one_node into a capability type.