std::unique_ptr's default constructor must be constexpr in order to allow constant initialization to take place for static objects; Even though we can never have a constexpr unique_ptr variable since it's not a literal type. This patch adds tests that constant initialization takes place by using the __attribute__((require_constant_initialization)) macro. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300158 91177308-0d34-0410-b5e6-96231b3b80d8