Make `& Class.foo` illegal

[Johan]

Currently this code is not rejected by the compiler and instead 
creates non-functioning (undefined) code:
```
struct S {
     void foo() {}
}

void main() {
     auto a = &S.foo;
}
```

See https://issues.dlang.org/show_bug.cgi?id=21195 .

Considering https://www.digitalmars.com/articles/b68.html, why 
not simply reject `&S.foo` during semantic analysis?

-Johan