Change value::parent to a value_ref_ptr

Change value::parent to a value_ref_ptr

This changes value::parent to a value_ref_ptr.  This removes a bit of
manual reference count management.

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

	* value.c (~value): Update.
	(struct value) <parent>: Now a value_ref_ptr.
	(value_parent, set_value_parent, value_address, value_copy):
	Update.