[test] Fix an XRay test on FreeBSD

Summary: Fixing clang-test on FreeBSD as a follow-up of https://reviews.llvm.org/D43378 to handle the revert happened in r325749.

Reviewers: devnexen, krytarowski, dberris

Subscribers: emaste, dberris, cfe-commits

Differential Revision: https://reviews.llvm.org/D45002

llvm-svn: 328797