[CUDA]Fix dynamic|guided scheduling.

The existing implementation of the dynamic scheduling
breaks the contract introduced by the original openmp
runtime and, thus, is incorrect. Patch fixes it and
introduces correct dynamic scheduling model.

Thanks to Alexey Bataev for submitting this patch.

Differential Revision: https://reviews.llvm.org/D47333



git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@333225 91177308-0d34-0410-b5e6-96231b3b80d8