OpenACC Github PR#499 defines the pqr-list as having at least 1 item. We
already handle that for all but 'wait', so this patch just does the work
to add it for 'wait', plus adds tests.
The arguments to this are the same as for the 'wait' clause, so this
reuses all of that infrastructure. So all this has to do is support a
pair of clauses that are already implemented (if and async), plus create
an AST node. This patch does so, and adds proper testing.