[XRay] Refcount backing store for buffers

Summary:
This change implements the ref-counting for backing stores associated
with generational buffer management. We do this as an implementation
detail of the buffer queue, instead of exposing this to the interface.

This change allows us to keep the buffer queue interface and usage model
the same.

Depends on D53551.

Reviewers: mboerger, eizan

Subscribers: jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D53560

llvm-svn: 345471