[OpenACC] Implement AST for OpenACC Compute Constructs (#81188)

'serial', 'parallel', and 'kernel' constructs are all considered
'Compute' constructs. This patch creates the AST type, plus the required
infrastructure for such a type, plus some base types that will be useful
in the future for breaking this up.

The only difference between the three is the 'kind'( plus some minor
 clause legalization rules, but those can be differentiated easily
enough), so rather than representing them as separate AST nodes, it
seems
to make sense to make them the same.

Additionally, no clause AST functionality is being implemented yet, as
that fits better in a separate patch, and this is enough to get the
'naked' constructs implemented.

This is otherwise an 'NFC' patch, as it doesn't alter execution at all,
so there aren't any tests.  I did this to break up the review workload
and to get feedback on the layout.