Experimantal dfsan mode "fast16labels=1"

Summary:
dfsan mode "fast16labels=1".
In this mode the labels are treated as 16-bit bit masks.

Reviewers: pcc

Reviewed By: pcc

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D62870

llvm-svn: 362859