Change varobj_iter::next to return unique_ptr

Change varobj_iter::next to return unique_ptr

This changes varobj_iter::next to return a unique_ptr.  This fits in
with the ongoing theme of trying to express these ownership transfers
via the type system.

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* varobj.c (update_dynamic_varobj_children): Update.
	* varobj-iter.h (struct varobj_iter) <next>: Change return type.
	* python/py-varobj.c (struct py_varobj_iter) <next>: Change return
	type.
	(py_varobj_iter::next): Likewise.