[clang][dataflow][NFC] Clarify guarantees on returned vector size for `runDataflowAnalysis`.

[ymandel]

Summary

Adjusts the comment to specify that the output vector's size matches the number of CFG blocks.

clang/include/clang/Analysis/FlowSensitive/DataflowAnalysis.h

template <typename LatticeT> struct DataflowAnalysisState {
   // Model of a program property.
   LatticeT Lattice;
 
   // Model of the state of the program (store and heap).
   Environment Env;
 };
 
 /// Performs dataflow analysis and returns a mapping from basic block IDs to
-/// dataflow analysis states that model the respective basic blocks. Indices
-/// of the returned vector correspond to basic block IDs. Returns an error if
+/// dataflow analysis states that model the respective basic blocks. The
+/// returned vector, if any, will have the same size as the number of CFG
+/// blocks, with indices corresponding to basic block IDs. Returns an error if
 /// the dataflow analysis cannot be performed successfully.
 template <typename AnalysisT>
 llvm::Expected<std::vector<
     llvm::Optional<DataflowAnalysisState<typename AnalysisT::Lattice>>>>
 runDataflowAnalysis(const ControlFlowContext &CFCtx, AnalysisT &Analysis,
                     const Environment &InitEnv) {
   auto TypeErasedBlockStates =
       runTypeErasedDataflowAnalysis(CFCtx, Analysis, InitEnv);