[XRay] Add atomic fences around non-atomic reads and writes

Summary:
We need these fences to ensure that other threads attempting to read
bytes in the buffer will see thw writes committed before the extents are
updated. Without these, the writes can be un-committed by the time the
buffer extents counter is updated -- the fences should ensure that the
records written into the log have completed by the time we observe the
buffer extents from different threads.

Reviewers: mboerger

Subscribers: jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D54291

llvm-svn: 346474