[msan] Block reports from interceptors during _Unwind_Backtrace

Summary:
I'm hitting a big recursive report from:
  uninit on strlen ->
  __msan::PrintWarningWithOrigin() ->
  __msan::GetStackTrace() ->
  __sanitizer::StackTrace::SlowUnwindStack() ->
  _Unwind_Backtrace() ->
  ... libgcc calls ->
  uninit on strlen() ->
  ... repeats

Reviewers: eugenis

Differential Revision: http://llvm-reviews.chandlerc.com/D497