This is once again simply enabling this for 'data', 'enter data', and
'exit data' (and ensuring we error for 'host_data'). Implementation is
very simply to enable it rather than emit the not-implemented
diagnostic.
Semantically this is identical to all other constructs with this tag,
except in this case the 'wait' and 'async' are the only ones allowed
after it. This patch implements that rule using the existing
infrastructure.
These constructs are all very similar and closely related, so this patch
creates the AST nodes for them, serialization, printing/etc.
Additionally the restrictions are all added as tests/todos in the tests,
as those will have to be implemented once we get those clauses implemented.
