[OpenMP] Prepare sema to support combined constructs with omp distribute and omp for

https://reviews.llvm.org/D32237

This patch prepares sema with additional fields to support all those composite and combined constructs of OpenMP that include pragma 'distribute' and 'for', such as 'distribute parallel for'. It also extends the regression tests for 'distribute parallel for' and adds a new one.

llvm-svn: 300802