[XRay][compiler-rt] Use llvm-xray in FDR mode tests

Summary:
This change allows us to do an end-to-end test of the FDR mode
implementation that uses the llvm-xray tooling to verify that what we
are both writing and reading the data in a consistent manner.

Reviewers: kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31454

llvm-svn: 299042