[XRay] Use more portable control block

Summary:
In D53560, we assumed a specific layout for memory without using an
explicit structure. This follow-up change uses more portable layout
control by using unions in a struct, and consolidating the memory
management code in the buffer queue.

We also take the opportunity to improve the documentation on the types
and operations, along with simplifying some of the logic in the buffer
queue implementation.

Reviewers: mboerger, eizan

Subscribers: jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D53802

llvm-svn: 345485