[clang][dataflow] Fix -Wrange-loop-construct in DataflowAnalysisContext.cpp (NFC)

/llvm-project/clang/lib/Analysis/FlowSensitive/DataflowAnalysisContext.cpp:247:10: note: use reference type 'const llvm::Sma
llVector<Atom> &' to prevent copying
    for (const llvm::SmallVector<Atom> Class : Info.EquivalentAtoms)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                       &
1 error generated.