[XRay][docs] Examples for how to use XRay

Summary:
This document is an attempt at showing how XRay could be used to debug
latency issues with LLVM tools, and how to use the llvm-xray tool to
analyse XRay traces.

Reviewers: echristo, mehdi_amini, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31493

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299133 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed