[OpenMP] set_bits iterator yields unsigned elements, no reference (NFC).

BitVector::set_bits() returns an iterator range yielding unsinged
elements, which always will be copied while const & gives the impression
that there will be no copy. Newer version of clang complain:

    warning: loop variable 'SetBitsIt' is always a copy because the range of type 'iterator_range<llvm::BitVector::const_set_bits_iterator>' (aka 'iterator_range<const_set_bits_iterator_impl<llvm::BitVector> >') does not return a reference [-Wrange-loop-analysis]

Reviewers: jdoerfert, rnk

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D77010