Add something similar to `Reflection::IndexOf<auto MemberPtr>`

[christianparpart]

supposed this:

struct S {
    int foo;
    std::string bar;
};

static_assert(Reflection::IndexOf<&S::foo> == 0);
static_assert(Reflection::IndexOf<&S::bar> == 1);

It would be nice to get this functionality to retrieve the index of a field by symbolic name.