[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<>.