[flang] Provide access to constant character array data

[klausler]

Summary

Allow direct access to constant character array data (for creating a hash ID of a constant).

[PeteSteinfeld]

Builds, tests, and looks good.

flang/include/flang/Evaluate/constant.h

public:
   ~Constant();
 
   bool operator==(const Constant &that) const {
     return shape() == that.shape() && values_ == that.values_;
   }
   bool empty() const;
   std::size_t size() const;
 
+  const Scalar<Result> &values() const { return values_; }
   ConstantSubscript LEN() const { return length_; }
 
   std::optional<Scalar<Result>> GetScalarValue() const {
     if (Rank() == 0) {
       return values_;
     } else {
       return std::nullopt;
     }