[OpenMP] Add function attribute for triggering data sharing.
The backend should only emit data sharing code for the cases where it is needed.
A new function attribute is used by Clang to enable data sharing only for the cases where OpenMP semantics require it and there are variables that need to be shared.
Reviewers: hfinkel, Hahnfeld, ABataev, carlo.bertolli, caomhin
Reviewed By: ABataev
Subscribers: cfe-commits, jholewinski
Differential Revision: https://reviews.llvm.org/D41123