Reset cancellation status for 'parallel', 'sections' and 'for' constracts.

Without this fix cancellation status for parallel, sections and for persists 
across construct boundaries.

Differential Revision: https://reviews.llvm.org/D31419

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@299434 91177308-0d34-0410-b5e6-96231b3b80d8