[OpenMP][libomptarget] Add support for close map modifier

Summary:
This patch adds support for the close map modifier.

The close map modifier will overwrite the unified shared memory requirement and create a device copy of the data.

Reviewers: ABataev, Hahnfeld, caomhin, grokos, jdoerfert, AlexEichenberger

Reviewed By: Hahnfeld, AlexEichenberger

Subscribers: guansong, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D65340

llvm-svn: 368488