[OpenACC] Implement 'gang' clause parsing.

The 'gang' clause takes a 'gang-arg-list', which is one of three 'tag'
values, followed by either an 'int-expr' or a 'size-expr', both of which
we already have parsing functions for.

The optional tag values are only slightly complicated, as one is a
keyword (static), so mild modifications needed to be made for that.