[OpenACC] Implement Default clause for Compute Constructs (#88135)

As a followup to my previous commits, this is an implementation of a
single clause, in this case the 'default' clause. This implements all
semantic analysis for it on compute clauses, and continues to leave it
rejected for all others (some as 'doesnt appertain', others as 'not
implemented' as appropriate).

This also implements and tests the TreeTransform as requested in the
previous patch.