message: Fix potential null dereference.
  This was crashing when the ClangImporter could not be built. It should
  never happen, but we might as well remove the potential null dereference.
diff: @@ -1379,7 +1379,7 @@ lldb::TypeSystemSP SwiftASTContext::CreateInstance(lldb::LanguageType language,
  if (result.second) {
  StreamString ss;
  module_sp->GetDescription(&ss, eDescriptionLevelBrief);
  - if (module_swift_ast->HasFatalErrors())
  + if (module_swift_ast && module_swift_ast->HasFatalErrors())
  ss << ": "
  << module_swift_ast->GetFatalErrors().AsCString("unknown error");
  