tsan: better report for bad mutex unlocks
+ fixes crashes due to races on symbolizer, see
https://code.google.com/p/thread-sanitizer/issues/detail?id=55



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@207206 91177308-0d34-0410-b5e6-96231b3b80d8