[PATCH][OpenACC] Don't error on implicitly private induction variables in gfortran

[Cesar Philippidis]

At present, the fortran FE reports an error if the user adds an explicit
private clause to an induction variable used by an acc loop. This patch
teaches the fortran acc block resolver how to cope with "duplicate"
private clauses, so that it doesn't error anymore.

Is this patch OK for trunk? I bootstrapped and regression tested it for
x86_64 with nvptx offloading.

Thanks,
Cesar