Use new_reference for struct value

Use new_reference for struct value

value_incref returned its argument just as a convenience, which in the
end turned out to only be used in precisely the cases where
new_reference helps.  So, this patch changes value_incref to return
void and changes some value-using code to use new_reference.

I also noticed that the comments for value_incref and value_decref
were swapped, so this patch fixes those.

ChangeLog
2018-04-30  Tom Tromey  <tom@tromey.com>

	* varobj.c (install_new_value): Use new_reference.
	* value.h (value_incref): Return void.  Swap intro comment with
	value_decref.
	* value.c (set_value_parent): Use new_reference.
	(value_incref): Return void.  Update intro comment.
	(release_value): Use new_reference.
	* dwarf2loc.c (dwarf2_evaluate_loc_desc_full): Use new_reference.