fix const-correctness of ConstantFlag::operator->

[metsw24-max]

Broken operator-> on ConstantFlag

The arrow operator is a const member yet returns &value as a plain T*; inside a const method that address has type const T*, so the conversion is ill-formed and any constantFlag->member fails to compile. It stayed hidden because this is the only operator-> on the class and the shipped test only reaches Get() through a ConstantFlag pointer, so the member operator is never instantiated.

Every other value type here already pairs a mutable T* overload with a const const T* one, and a constant only needs the const form, so returning const T* lines it up with the rest. The test now reaches into a struct-typed constant through the arrow, which fails to build on master and passes with the change.

[Taywee]

Good call, and good fix. It makes sense to me.

Thanks for the PR.