[OpenACC] Implement 'reduction' for combined constructs.

Once again, this is a clause on a combined construct that does almost
exactly what the loop/compute construct version does, only with some sl
ightly different evaluation rules/sema rules as it doesn't have to
consider the parent, just the 'combined' construct.  The two sets of
rules for reduction on loop and compute are fine together, so this
ensures they are all enforced for this too.

The 'gangs' 'num_gangs' 'reduction' diagnostic (Dim>1) had to be applied
to num_gangs as well, as it previously wasn't permissible to get in this
situation, but we now can.

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