[flang][Evaluate] Fix AsGenericExpr for Relational (#138455)

The variant in Expr<Type<TypeCategory::Logical, KIND>> only contains
Relational<SomeType>, not other, more specific Relational<T> types.

When calling AsGenericExpr for a value of type Relational<T>, the AsExpr
function will attempt to create Expr<> directly for Relational<T>, which
won't work for the above reason.

Implement an overload of AsExpr for Relational<T>, which will wrap the
Relational<T> in Relational<SomeType> before creating Expr<>.

Author: kparzysz

flang/include/flang/Evaluate/tools.h

@@ -130,6 +130,14 @@ template <typename A> common::IfNoLvalue<Expr<ResultType<A>>, A> AsExpr(A &&x) {
   return Expr<ResultType<A>>{std::move(x)};
 }
 
+template <typename T, typename U = typename Relational<T>::Result>
+Expr<U> AsExpr(Relational<T> &&x) {
+  // The variant in Expr<Type<TypeCategory::Logical, KIND>> only contains
+  // Relational<SomeType>, not other Relational<T>s. Wrap the Relational<T>
+  // in Relational<SomeType> before creating Expr<>.
+  return Expr<U>(Relational<SomeType>{std::move(x)});
+}
+
 template <typename T> Expr<T> AsExpr(Expr<T> &&x) {
   static_assert(IsSpecificIntrinsicType<T>);
   return std::move(x);