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