[XRay][profiler] Part 5: Profiler File Writing
This is part of the larger XRay Profiling Mode effort.
This patch implements the profile writing mechanism, to allow profiles
collected through the profiler mode to be persisted to files.
Follow-on patches would allow us to load these profiles and start
converting/analysing them through the llvm-xray tool.
Depends on D44620.
Reviewers: echristo, kpw, pelikan
Reviewed By: kpw
Differential Revision: https://reviews.llvm.org/D45998