[OpenMP][libomptarget] Add data sharing support in libomptarget

Summary: This patch extends the libomptarget functionality in patch D14254 with support for the data sharing scheme for supporting implicitly shared variables. The runtime therefore maintains a list of references to shared variables.

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

Reviewed By: Hahnfeld, grokos

Subscribers: guansong, llvm-commits, openmp-commits

Differential Revision: https://reviews.llvm.org/D41485

llvm-svn: 324495