[esan|wset] Add 8-level working set snapshot accumulation

Summary:
Adds a new option -snapshot_step controlling the frequency distribution for
an 8-level series of samples using each bit of each shadow byte.
Implements accumulation from each level to the next higher level at the
specified frequency.

Adds storage of the 8 series of samples using CircularBuffer instances.
Fixes an error in the circular buffer data structure where a static
object's destructor will be called too early.

Prints the results out at the end in a simple manner to give us something
to start with.

Updates the workingset-samples test to test the new feature.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D20833

llvm-svn: 271683