[OpenMP] Add OpenMP data sharing infrastructure using global memory

Summary:
This patch handles the Clang code generation phase for the OpenMP data sharing infrastructure.

TODO: add a more detailed description.

Reviewers: ABataev, carlo.bertolli, caomhin, hfinkel, Hahnfeld

Reviewed By: ABataev

Subscribers: jholewinski, guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D43660

llvm-svn: 327513