[OpenACC] Implement auto/seq/independent clause Sema for 'loop'

These three clauses are all quite trivial, as they take no parameters.
They are mutually exclusive, and 'seq' has some other exclusives that
are implemented here.

The ONE thing that isn't implemented is 2.9's restriction (line 2010):
  'A loop associated with a 'loop' construct that does not have a 'seq'
   clause must be written to meet all the following conditions'.

Future clauses will require similar work, so it'll be done as a
followup.