add debug code to chase down a rare crash in asan/lsan...

add debug code to chase down a rare crash in asan/lsan https://github.com/google/sanitizers/issues/1193

Summary: add debug code to chase down a rare crash in asan/lsan https://github.com/google/sanitizers/issues/1193

Reviewers: vitalybuka

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D80967