[OpenMP][libomptarget] Add support for declare target to clause under unified memory

Summary:
This patch adds support for handling variables under the:

```
#pragma omp declare target to()
```

clause when the 

```
#pragma omp requires unified_shared_memory
```

is used.

The address of the host variable is copied into the device pointer just like for the declare target link case.

Reviewers: ABataev, caomhin, grokos, AlexEichenberger

Reviewed By: grokos

Subscribers: jcownie, guansong, jdoerfert, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D63106

llvm-svn: 363825