[esan|wset] EfficiencySanitizer working set tool fastpath
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.
Subscribers: vitalybuka, zhaoqin, kcc, eugenis, llvm-commits
Differential Revision: http://reviews.llvm.org/D20483