[OpenMP][libomptarget] Enable requires flags for target libraries.

Summary:
Target link variables are currently implemented by creating a copy of the variables on the device side and unified memory never gets exploited.

When the prgram uses the:

```
#pragma omp requires unified_shared_memory
```

directive in conjunction with a declare target link, the linked variable is no longer allocated on the device and the host version is used instead.

This behavior is overridden by performing an explicit mapping.

A Clang side patch is required.

Reviewers: ABataev, AlexEichenberger, grokos, Hahnfeld

Reviewed By: AlexEichenberger, grokos, Hahnfeld

Subscribers: Hahnfeld, jfb, guansong, jdoerfert, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D60223

llvm-svn: 361294