[PATCH] [ASan] Introduce the dump_instruction_bytes flag to print the faulting instruction upon SIGSEGV

[Alexander Potapenko]

I take back my argument about crashes in _Unwind_Backtrace.
Apparently at least on OSX unwinding the stack from the unmapped page doesn't necessarily crash the unwinder. Thus we probably need to unprotect the faulting page before trying to dump instruction bytes.

http://reviews.llvm.org/D5167