CrashHandler: be careful about crashing while handling

Summary:
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.

<rdar://problem/53503334>

Subscribers: hiraditya, jkorous, dexonsmith, llvm-commits, beanz, Bigcheese, thakis, lattner, jordan_rose

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65235

llvm-svn: 367031