[clang][OpenMP] Fix build when using libgomp

Summary: rL356570 introduced a test which only passes with the default openmp library, libomp, and fails with other openmp libraries, such as libgomp. Explicitly choose libomp.

Reviewers: lebedev.ri

Subscribers: guansong, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59609

llvm-svn: 356614