Add tests that std::unique_ptr's default constructor is constexpr.

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.

llvm-svn: 300158