[OPENMP]Fix DSA for loop iteration variables in simd loops.

According to the OpenMP 5.0 standard, the loop iteration variable in the associated
for-loop of a simd construct with just one associated for-loop may be
listed in a private, lastprivate, or linear clause with a linear-step
that is the increment of the associated for-loop. Also, the loop
teration variables in the associated for-loops of a simd construct with
multiple associated for-loops may be listed in a private or lastprivate
clause.

llvm-svn: 364650