async just takes an integral value, but it has a little bit of special
rules in sema, so it is implemented slightly differently than int-expr.
This patch implements async parsing.
The 'vector_length' clause is the first of the 'int-expr' clauses that I've
implemented. Currently this is just being parsed as an assignment-expr,
since it needs to be usable in a list. Sema implementation will
enforce the integral-nature of it.
'bind' takes either a string literal, or an 'identifier' representing
the device-side function that this routine is intended to 'bind' to
(that is, to call). However, it seems that the intent is to permit the
'identifier' to reference any function, thus we're implementing this as
an ID expression.
Additionally, while working on this I discovered that the 'routine' ID
expression parsing for C++ wouldn't allow non-static member functions to
be referenced since it expected us to call it, this was fixed as a part
of this patch as the 'bind' support needed it too. A test was added for
routine.
The 'collapse' clause takes an optional 'force:' followed by an integer
constant expression. For now, parse this as an assignment expression,
and we'll check it for value/ICE later.
