Add function to get the number of DFSan labels allocated.

Expose the number of DFSan labels allocated by adding function dfsan_get_label_count().

Patch by Sam Kerner!

Differential Revision: http://llvm-reviews.chandlerc.com/D3109

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204854 91177308-0d34-0410-b5e6-96231b3b80d8