Change varobj_item::value to a value_ref_ptr

Change varobj_item::value to a value_ref_ptr

This changes varobj_item::value to be a value_ref_ptr, removing some
manual management.

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

	* varobj.c (install_dynamic_child, varobj_clear_saved_item)
	(update_dynamic_varobj_children, create_child)
	(create_child_with_value): Update.
	* varobj-iter.h (struct varobj_item) <value>: Now a
	value_ref_ptr.
	* python/py-varobj.c (py_varobj_iter::next): Call release_value.