polly: update for LLVM API change

SVN r209103 removed the OwningPtr variant of the MemoryBuffer APIs.  Switch to
the equivalent std::unique_ptr versions.  This should clear up the build bots.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@209104 91177308-0d34-0410-b5e6-96231b3b80d8