[XRay][compiler-rt] Use a hand-written circular buffer in BufferQueue

Summary:
This change removes the dependency on using a std::deque<...> for the
storage of the buffers in the buffer queue. We instead implement a
fixed-size circular buffer that's resilient to exhaustion, and preserves
the semantics of the BufferQueue.

We're moving away from using std::deque<...> for two reasons:

  - We want to remove dependencies on the STL for data structures.

  - We want the data structure we use to not require re-allocation in
    the normal course of operation.

The internal implementation of the buffer queue uses heap-allocated
arrays that are initialized once when the BufferQueue is created, and
re-uses slots in the buffer array as buffers are returned in order.

We also change the lock used in the implementation to a spinlock
instead of a blocking mutex. We reason that since the release operations
now take very little time in the critical section, that a spinlock would
be appropriate.

This change is related to D38073.

This change is a re-submit with the following changes:

  - Keeping track of the live buffers with a counter independent of the
    pointers keeping track of the extents of the circular buffer.

  - Additional documentation of what the data members are meant to
    represent.

Reviewers: dblaikie, kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38119

llvm-svn: 314877

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