[flang][Evaluate] Add ExtractSubstring to evaluate/tools.h

There are utility functions to extract a DataRef, optionally
traversing into a substring, but there is no function to get
the subscript itself. This patch adds one.

Author: kparzysz

flang/include/flang/Evaluate/tools.h

@@ -430,6 +430,28 @@ template <typename A> std::optional<CoarrayRef> ExtractCoarrayRef(const A &x) {
   }
 }
 
+struct ExtractSubstringHelper {
+  template <typename T> static std::optional<Substring> visit(T &&) {
+    return std::nullopt;
+  }
+
+  static std::optional<Substring> visit(const Substring &e) { return e; }
+
+  template <typename T>
+  static std::optional<Substring> visit(const Designator<T> &e) {
+    return std::visit([](auto &&s) { return visit(s); }, e.u);
+  }
+
+  template <typename T>
+  static std::optional<Substring> visit(const Expr<T> &e) {
+    return std::visit([](auto &&s) { return visit(s); }, e.u);
+  }
+};
+
+template <typename A> std::optional<Substring> ExtractSubstring(const A &x) {
+  return ExtractSubstringHelper::visit(x);
+}
+
 // If an expression is simply a whole symbol data designator,
 // extract and return that symbol, else null.
 template <typename A> const Symbol *UnwrapWholeSymbolDataRef(const A &x) {