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

Reviewers: eugenis

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1406
DataFlowSanitizer: Add a debugging feature to help us track nonzero labels.

Summary:
When the -dfsan-debug-nonzero-labels parameter is supplied, the code
is instrumented such that when a call parameter, return value or load
produces a nonzero label, the function __dfsan_nonzero_label is called.
The idea is that a debugger breakpoint can be set on this function
in a nominally label-free program to help identify any bugs in the
instrumentation pass causing labels to be introduced.

Reviewers: eugenis

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1405