[OpenMP] Add test for requires and unified shared memory clause with declare target link
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
Differential Revision: https://reviews.llvm.org/D62407