Update to reflect changes in LLVM.

Last uses of OwningPtr from llvm where removed in r209103.  Replaced 
llvm::OwningPtr with std::unique_ptr.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@547 812d1f90-f944-d88a-411a-122bdc93e1ea