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.
The 'if_present' clause controls the replacement of addresses in the
var-list in current device memory. This clause can only go on
'host_device'. From a Sema perspective, there isn't anything to do
beyond add this to AST and pass it on.