[XRay][compiler-rt] XRay Buffer Queue

Summary:
This implements a simple buffer queue to manage a pre-allocated queue of
fixed-sized buffers to hold XRay records. We need this to support
Flight Data Recorder (FDR) mode. We also implement this as a sub-library
first to allow for development before actually using it in an
implementation.

Some important properties of the buffer queue:

- Thread-safe enqueueing/dequeueing of fixed-size buffers.
- Pre-allocation of buffers at construction.

Reviewers: majnemer, rSerge, echristo

Subscribers: mehdi_amini, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D26232

llvm-svn: 287910