message: Add string to llvm_unreachable.
diff: @@ -1953,7 +1953,7 @@ static lldb::TypeSystemSP CreateTypeSystemInstance(lldb::LanguageType language,
  assert(!module);
  return SwiftASTContext::CreateInstance(language, *target, extra_options);
  }
  - llvm_unreachable();
  + llvm_unreachable("Neither type nor module given to CreateTypeSystemInstance");
  }
  void SwiftASTContext::Initialize() {
  