[OpenMP] Add test for requires and unified shared memory clause with declare target link

Summary:
This patch adds a test for requires with unified share memory clause when a declare target link is present.

This test needs to go in prior to changes to declare target link for comparison purposes.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: guansong, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62407

llvm-svn: 361658