Remove free_value_chain

Remove free_value_chain

This patch changes value_release_to_mark and fetch_subexp_value to
return a std::vector of value references, rather than relying on the
"next" field that is contained in a struct value.  This makes it
simpler to reason about the returned values, and also allows for the
removal of free_value_chain.

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

	* value.h (fetch_subexp_value, value_release_to_mark): Update.
	(free_value_chain): Remove.
	* value.c (free_value_chain): Remove.
	(value_release_to_mark): Return a std::vector.
	* ppc-linux-nat.c (num_memory_accesses): Change "chain" to a
	std::vector.
	(check_condition): Update.
	* eval.c (fetch_subexp_value): Change "val_chain" to a
	std::vector.
	* breakpoint.c (update_watchpoint): Update.
	(can_use_hardware_watchpoint): Change "vals" to a std::vector.