openacc: Warn about "independent" "kernels" loops with data-dependences

openacc: Warn about "independent" "kernels" loops with data-dependences

This commit concerns loops in OpenACC "kernels" region that have been marked
up with an explicit "independent" clause by the user, but for which Graphite
found data dependences.  A discussion on the private internal OpenACC mailing
list suggested that warning the user about the dependences woud be a more
acceptable solution than reverting the user's decision. This behavior is
implemented by the present commit.

gcc/ChangeLog:

        * common.opt: Add flag Wopenacc-false-independent.
        * omp-offload.cc (oacc_loop_warn_if_false_independent): New function.
        (oacc_loop_fixed_partitions): Call from here.