[OpenMP] Avoid emitting maps for target link variables when unified memory is used

Summary: This patch avoids the emission of maps for target link variables when unified memory is present.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: guansong, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60883

llvm-svn: 363435