[NFC][OpenMP] Add new test for parallel for code generation.

Summary:
This is a simple test of the parallel for code generation. It will be used to showcase the change introduced by patch D53443.


Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D53772

llvm-svn: 345417