[OpenMP] Initialize data sharing stack for SPMD case

Summary: In the SPMD case, we need to initialize the data sharing and globalization infrastructure. This covers the case when an SPMD region calls a function in a different compilation unit.

Reviewers: ABataev, carlo.bertolli, caomhin

Reviewed By: ABataev

Subscribers: Hahnfeld, jholewinski, guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D49188

llvm-svn: 337015