Remove value::next and value::released

Remove value::next and value::released

This patch converts all_values to simply hold a list of references to
values.  Now, there's no need to have a value record whether or not it
is released -- there is only a single reference-counting mechanism for
values.  So, this also removes value::next, value::released, and
value_next.

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

	* value.c (struct value) <released, next>: Remove.
	(all_values): Now a std::vector.
	(allocate_value_lazy): Update.
	(value_next): Remove.
	(value_mark, value_free_to_mark, release_value)
	(value_release_to_mark): Update.