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 'gang' clause is used to specify parallel execution of loops, thus
has some complicated rules depending on the 'loop's associated compute
construct. This patch implements all of those.
