unique_ptrify thep passing of BugReports to BugReportEquivClass

I suspect llvm::ilist should take elements by unique_ptr, since it does
take ownership of the element (by stitching it into the linked list) -
one day.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216761 91177308-0d34-0410-b5e6-96231b3b80d8