unique_ptrify PathDiagnosticConsumer::HandlePathDiagnostic

FoldingSet, another intrusive data structure that could use some
unique_ptr love on its interfaces. Eventually.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216764 91177308-0d34-0410-b5e6-96231b3b80d8