After implementing 'loop', we determined that the link to its parent
only ever uses the type, not the construct itself. This patch removes
it, as it is both a waste and causes problems with serialization.
OpenACC restricts the contents of a 'for' loop affected by a 'loop'
construct without a 'seq'. The loop variable must be integer, pointer,
or random-access-iterator, it must monotonically increase/decrease, and
the trip count must be computable at runtime before the function.
This patch tries to implement some of these limitations to the best of
our ability, though it causes us to be perhaps overly restrictive at the
moment. I expect we'll revisit some of these rules/add additional
supported forms of loop-variable and 'monotonically increasing' here,
but the currently enforced rules are heavily inspired by the OMP
implementation here.
The worker clause specifies iterations of the loop/ that are executed in
parallel by distributing the iterations among the multiple works within
a single gang.
The sema rules for this type are simply that it cannot be combined with
a `kernel` construct with a `num_workers` clause, child `loop` clauses
cannot contain a `gang` or `worker` clause, and that the argument is oly
allowed when associated with a `kernel`.
