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.
As a first step in adding clause support for OpenACC to Semantic
Analysis, this patch adds the 'base' AST nodes required for clauses.
This patch has no functional effect at the moment, but followup patches
will add the semantic analysis of clauses (plus individual clauses).
