Don't use llvm::EnablePrettyStackTrace on macOS.

LLDB.framework gets loaded into Xcode and other 
frameworks, and this is inserting a signal handler into
the process even when lldb isn't used.  I have a bunch
of reports of this SignalHandler blowing out the stack,
which renders crash reports for the crash useless.

And in any case libraries really shouldn't be installing
signal handlers.

I only turned this off for APPLE platforms, I'll let
the maintainers of other platforms decide what policy
they want to have w.r.t. this.

llvm-svn: 319598