[OPENMP 4.0] Codegen for 'omp cancel' directive.

Add the next codegen for 'omp cancel' directive:
if (__kmpc_cancel()) {
  __kmpc_cancel_barrier();
  <exit construct>;
}


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241429 91177308-0d34-0410-b5e6-96231b3b80d8