Prepare PrettyStackTrace for LLDB adoption
This patch fixes the linkage for __crashtracer_info__, making it have the proper mangling (extern "C") and linkage (private extern).
It also adds a new PrettyStackTrace type, allowing LLDB to adopt this instead of Host::SetCrashDescriptionWithFormat().
Without this patch, CrashTracer on macOS won't pick up pretty stack traces from any LLVM client.
An LLDB commit adopting this API will follow shortly.
Differential Revision: https://reviews.llvm.org/D27683
llvm-svn: 289689