[Issue 21218] dtoh: protection attributes should be emitted to headers

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21218

Seb <greeenify at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|dtoh: protected attribute   |dtoh: protection attributes
                   |should be emitted to        |should be emitted to
                   |headers                     |headers

--- Comment #1 from Seb <greeenify at gmail.com> ---
This also applies to `private` and likely all other protection attributes:

```
extern(C++) struct Foo {
  private int a = 1;
}
```

generates currently:

```
struct Foo
{
    int32_t a;
    Foo() {}
};
```

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