[OpenACC] Enable 'self' sema for 'update' construct
The 'self' clause is an unfortunately difficult one, as it has a
significantly different meaning between 'update' and the other
constructs. This patch introduces a way for the 'self' clause to work
as both. I considered making this two separate AST nodes (one for
'self' on 'update' and one for the others), however this makes the
automated macros/etc for supporting a clause break.
Instead, 'self' has the ability to act as either a condition or as a
var-list clause. As this is the only one of its kind, it is implemented
all within it. If in the future we have more that work like this, we
should consider rewriting a lot of the macros that we use to make
clauses work, and make them separate ast nodes.
