[XRay][compiler-rt] Add an end-to-end test for FDR Logging

Summary:
This change exercises the end-to-end functionality defined in the FDR
logging implementation. We also prepare for being able to run traces
generated by the FDR logging implementation from being analysed with the
llvm-xray command that comes with the LLVM distribution.

This also unblocks D31385, D31384, and D31345.

Reviewers: kpw, pelikan

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D31452

llvm-svn: 298977