Add a polymorphic_ptr<T> smart pointer data type. It's a somewhat silly
unique ownership smart pointer which is *deep* copyable by assuming it
can call a T::clone() method to allocate a copy of the owned data.

This is mostly useful with containers or other collections of uniquely
owned data in C++98 where they *might* copy. With C++11 we can likely
remove this in favor of move-only types and containers wrapped around
those types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194315 91177308-0d34-0410-b5e6-96231b3b80d8