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

lldb/source/Initialization/SystemInitializerCommon.cpp

@@ -69,7 +69,9 @@ void SystemInitializerCommon::Initialize() {
   }
 #endif
 
+#if not defined(__APPLE__)
   llvm::EnablePrettyStackTrace();
+#endif
   Log::Initialize();
   HostInfo::Initialize();
   static Timer::Category func_cat(LLVM_PRETTY_FUNCTION);