[OpenMP] Add support for handling declare target to clause when unified memory is required

Summary:
This patch adds support for the handling of the variables under the declare target to clause.

The variables in this case are handled like link variables are. A pointer is created on the host and then mapped to the device. The runtime will then copy the address of the host variable in the device pointer.

Reviewers: ABataev, AlexEichenberger, caomhin

Reviewed By: ABataev

Subscribers: guansong, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63108

llvm-svn: 363959