State immunity — Jurisdictional immunity — Head of State — Whether acts performed outside scope of official duties attract immunity as acts of instrumentality of State — Acts in violation of international law — Claim by alien for wrongful death resulting from torture by security forces in foreign State — Whether international law forms part of substantive domestic law providing basis for jurisdiction of United States courts — United States Alien Tort Act — Whether providing cause of action for violation of international law or purely jurisdictional — United States Foreign Sovereign Immunities Act — The law of the United States