Remove a use of raw pointer ownership (then non-ownership) in TrimmedGraph::popNextReportGraph

(just cleaning up unique_ptr stuff by finding interesting 'reset' calls
at the moment)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217210 91177308-0d34-0410-b5e6-96231b3b80d8