Initial impl of repr_packed_without_abi

[lukaslueg]

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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[bors]

☔ The latest upstream changes (presumably #13286) made this pull request unmergeable. Please resolve the merge conflicts.