[dfsan] Runtime support for -dfsan-debug-nonzero-labels feature.

Reviewers: eugenis

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1406

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@188471 91177308-0d34-0410-b5e6-96231b3b80d8