Include SmallSet.h in BackendUtil.cpp

BackendUtil.cpp uses llvm::SmallSet but did not include the header. It was
included indirectly, but this will change once the AssumptionCache is removed.
NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289752 91177308-0d34-0410-b5e6-96231b3b80d8