message: Return 'unknown error' when error string is empty
diff: @@ -1311,7 +1311,8 @@ lldb::TypeSystemSP SwiftASTContext::CreateInstance(lldb::LanguageType language,
  StreamString ss;
  module_sp->GetDescription(&ss, eDescriptionLevelBrief);
  if (module_swift_ast->HasFatalErrors())
  - ss << ": " << module_swift_ast->GetFatalErrors().AsCString();
  + ss << ": "
  + << module_swift_ast->GetFatalErrors().AsCString("unknown error");
  target.GetDebugger().GetErrorFile()->Printf(
  "warning: Swift error in module %s.\n"
  