[flang] Provide access to constant character array data

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

Differential Revision: https://reviews.llvm.org/D101208

Author: klausler

flang/include/flang/Evaluate/constant.h

@@ -168,6 +168,7 @@ class Constant<Type<TypeCategory::Character, KIND>> : public ConstantBounds {
   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 {