the 'tile' clause requires that it be followed by N (where N is the
number of size expressions) 'tightly nested loops'. This means the
same as it does in 'collapse', so much of the implementation is
simliar/shared with that.
The 'tile' clause shares quite a bit of the rules with 'collapse', so a
followup patch will add those tests/behaviors. This patch deals with
adding the AST node.
The 'tile' clause takes a series of integer constant expressions, or *.
The asterisk is now represented by a new OpenACCAsteriskSizeExpr node,
else this clause is very similar to others.