This is a clause that is only valid on 'host_data' constructs, and
identifies variables which it should use the current device address.
From a Sema perspective, the only thing novel here is mild changes to
how ActOnVar works for this clause, else this is very much like the rest
of the 'var-list' clauses.
This is another new clause specific to 'exit data' that takes a pointer
argument. This patch implements this the same way we do a few other
clauses (like attach) that have the same restrictions.