Bug fix for hang when tasks used in nested parallel

Bug fix for hang when omp task and nested parallelism used together.
Still some problem remains with task state saving/restoring, but
user's case works fine now. All tasking unit tests passed as well.

Patch by Andrey Churbanov

Differential Revision: http://reviews.llvm.org/D21558


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@273297 91177308-0d34-0410-b5e6-96231b3b80d8