CrashHandler: be careful about crashing while handling
Looking at the current Apple-specific code for crash handling it does a few
silly things that I think we should avoid while handling crashes:
- Try real hard not to allocate.
- Set the global crash reporter string early so that any crash while generating the stack trace will still report some info.
- Prevent reordering of operations in the current thread.
Subscribers: hiraditya, jkorous, dexonsmith, llvm-commits, beanz, Bigcheese, thakis, lattner, jordan_rose
Differential Revision: https://reviews.llvm.org/D65235