[clang][dataflow] Mark `getDeclCtx` function in dataflow `Environment` `const`.

clang/include/clang/Analysis/FlowSensitive/DataflowEnvironment.h

public:
   void addToFlowCondition(BoolValue &Val);
 
   /// Returns true if and only if the clauses that constitute the flow condition
   /// imply that `Val` is true.
   bool flowConditionImplies(BoolValue &Val) const;
 
   /// Returns the `DeclContext` of the block being analysed, if any. Otherwise,
   /// returns null.
-  const DeclContext *getDeclCtx() { return CallStack.back(); }
+  const DeclContext *getDeclCtx() const { return CallStack.back(); }
 
   /// Returns whether this `Environment` can be extended to analyze the given
   /// `Callee` (i.e. if `pushCall` can be used), with recursion disallowed and a
   /// given `MaxDepth`.
   bool canDescend(unsigned MaxDepth, const DeclContext *Callee) const;
 
   /// Returns the `ControlFlowContext` registered for `F`, if any. Otherwise,
   /// returns null.