[XRAY] [x86_64] Adding a Flight Data filetype reader to the llvm-xray Trace implementation.

Summary:
The file type packs function trace data onto disk from potentially multiple
threads that are aggregated and flushed during the course of an instrumented
program's runtime.

It is named FDR mode or Flight Data recorder as an analogy to plane
blackboxes, which instrument a running system without access to IO.

The writer code is defined in compiler-rt in xray_fdr_logging.h/cc

Reviewers: rSerge, kcc, dberris

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29697

llvm-svn: 295397