[esan|wset] EfficiencySanitizer working set tool fastpath

Summary:
Adds fastpath instrumentation for esan's working set tool.  The
instrumentation for an intra-cache-line load or store consists of an
inlined write to shadow memory bits for the corresponding cache line.

Adds a basic test for this instrumentation.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D20483

llvm-svn: 270640