Change varobj to use value_ref_ptr

Change varobj to use value_ref_ptr

This changes varobj to use value_ref_ptr, allowing the removal of some
manual reference count management.

gdb/ChangeLog
2018-04-06  Tom Tromey  <tom@tromey.com>

	* varobj.h (struct varobj) <value>: Now a value_ref_ptr.
	* varobj.c (varobj_set_display_format, varobj_set_value)
	(install_default_visualizer, construct_visualizer)
	(install_new_value, ~varobj, varobj_get_value_type)
	(my_value_of_variable, varobj_editable_p): Update.
	* c-varobj.c (c_describe_child, c_value_of_variable)
	(cplus_number_of_children, cplus_describe_child): Update.
	* ada-varobj.c (ada_number_of_children, ada_name_of_child)
	(ada_path_expr_of_child, ada_value_of_child, ada_type_of_child)
	(ada_value_of_variable, ada_value_is_changeable_p): Update.