[Clang] Fix 'counted_by' for nested struct pointers (#110497)

Fix counted_by attribute for cases where the flexible array member is
accessed through struct pointer inside another struct:

```
struct variable {
        int a;
        int b;
        int length;
        short array[] __attribute__((counted_by(length)));
};

struct bucket {
        int a;
        struct variable *growable;
        int b;
};
```

__builtin_dynamic_object_size(p->growable->array, 0);

This commit makes sure that if the StructBase is both a MemberExpr and a
pointer, it is treated as a pointer. Otherwise clang will generate to
code to access the address of p->growable intead of loading the value of
p->growable->length.

Fixes #110385