[esan] Add circular buffer data structure

Summary:
Adds a new class, CircularBuffer, for holding a wrap-around fixed-size
sequence of a primitive data type.  This will be used initially by the
working set tool.

Adds a unit test for CircularBuffer, including infrastructure support to
include esan headers and to link with the esan library by pretending to
want the working set tool.

Reviewers: aizatsky, filcab

Subscribers: vitalybuka, zhaoqin, kcc, eugenis, llvm-commits, kubabrecka

Differential Revision: http://reviews.llvm.org/D20579

llvm-svn: 271286