[flang] Fix use-after-free cases found by valgrind (llvm#122394)

The expression traversal library needs to use interfaces into triplets
(and substrings) that return pointers to nested expressions, rather than
optional copies of them, since at least one semantic analysis collects a
set of references to some subexpression representation class instances,
and those references obviously can't point to local copies of objects.

Fixes llvm#121999.

flang/include/flang/Evaluate/traverse.h

@@ -139,7 +139,7 @@ class Traverse {
     return visitor_(x.base());
   }
   Result operator()(const Triplet &x) const {
-    return Combine(x.lower(), x.upper(), x.stride());
+    return Combine(x.GetLower(), x.GetUpper(), x.GetStride());
   }
   Result operator()(const Subscript &x) const { return visitor_(x.u); }
   Result operator()(const ArrayRef &x) const {
@@ -151,7 +151,7 @@ class Traverse {
   }
   Result operator()(const DataRef &x) const { return visitor_(x.u); }
   Result operator()(const Substring &x) const {
-    return Combine(x.parent(), x.lower(), x.upper());
+    return Combine(x.parent(), x.GetLower(), x.GetUpper());
   }
   Result operator()(const ComplexPart &x) const {
     return visitor_(x.complex());

flang/include/flang/Evaluate/variable.h

@@ -331,8 +331,14 @@ class Substring {
   }
 
   Expr<SubscriptInteger> lower() const;
+  const Expr<SubscriptInteger> *GetLower() const {
+    return lower_.has_value() ? &lower_->value() : nullptr;
+  }
   Substring &set_lower(Expr<SubscriptInteger> &&);
   std::optional<Expr<SubscriptInteger>> upper() const;
+  const Expr<SubscriptInteger> *GetUpper() const {
+    return upper_.has_value() ? &upper_->value() : nullptr;
+  }
   Substring &set_upper(Expr<SubscriptInteger> &&);
   const Parent &parent() const { return parent_; }
   Parent &parent() { return parent_; }