[OPENMP52] Support Support omp_cur_iteration modifier for doacross

clause.

This is just syntax to make it easier for the user. It doesn't add any
new functionality.

for
doacross(sink: omp_cur_iteration - 1)
Equivalent to
doacross(sink: ConterVar - 1, ...)

doacross(source: omp_cur_iteration)
Equivalent to
doacross(source)

And restriction is:
OMP5.2 p.327

If vector is specified with the omp_cur_iteration keyword and with
sink as the dependence-type then it must be omp_cur_iteration - 1.

If vector is specified with source as the dependence-type then it must be
omp_cur_iteration.

Differential Revision: https://reviews.llvm.org/D154556