[OpenMP] Add function attribute for triggering data sharing.

[OpenMP] Add function attribute for triggering data sharing.

Summary:
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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320527 91177308-0d34-0410-b5e6-96231b3b80d8