Initial impl of `repr_packed_without_abi` (#13398)

Fixes #13375

I've added the lint next to the other attribute-related ones. Not sure
if this is the correct place, since while we are looking after the
`packed`-attribute (there is nothing we can do about types defined
elsewhere), we are more concerned about the type's representation set by
the attribute (instead of "duplicate attributes" and such).

The lint simply looks at the attributes themselves without concern for
the item-kind, since items where `repr` is not allowed end up in a
compile-error anyway.

I'm somewhat concerned about the level of noise this lint would cause
if/when it goes into stable, although it does _not_ come up in
`lintcheck`.

```
changelog: [`repr_packed_without_abi`]: Initial implementation
```

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